HHO is the gas resulting from electrolysis of water. Two H's and an O when seperated are very flamable. On top of being abundant it is cheap and cleaner burning than gasoline. Operating on purley HHO requires lots of modifications to a car but suplementing your gasoline with HHO requires little. There are a million sites out there on HHO car conversion kits. All of them are building plans for anywhere between 50 to 500 dollars. It seems odd that there is no instructable on it. So here, for free, is my attempt at making one. The text at the top is my first try and the text under the "-" line is my second try.

Step 1: Tools and Materials

This page ended up changing through the process and can vary depending on how you choose to improve upon the design I used.
Tools:

knife or strippers
soddering iron(not essential but saves time)
screw driver
voltage tester(depends)
*drill
*#7 bit
*1/4" tap
*may change depending on the electrode you use and how you connect it to the wire
Materials:(Keep in mind all the materials I used are "borrowed" from work or I had around the house so they won't be the most effective solutions. Be creative with what you use)

Plastic container (I used a grape juice bottle)
High temperature silicone sealant <-expensive; regular silicone sealant MAY work
Wire (I used some left over wire from the amp that went in my wife's car, 14awg)
Teflon tape
Plastic tubing (I had to siphon gas once to change the fuel pump and had this hose from it, not sure why I kept it but I'm glad I did)
salt (for an electrolyte in the water)
Stainless Steel Electrode(update: THIS WILL NOT WORK)
I put this one on bottom because it will have the biggest explination. I am told stainless isn't esential but will not corrode like other metals. I also read that coiled wire would be the best type(i.e. pipe, plate, wire).Platinum would be your number 1 choice but who can afford that? I used some pipe I found at work.

-updates-
-I ended up using the graphite from 2 pencils as my electrodes

Step 2: Plan Your Build

Before you start cutting into your own grape juice containers and sealing be sure it fits in your car. Find a spot to cram your plastic container into. It needs to be away from direct conact to the motor. Near the battery or air filter is preferable. I got lucky that my battery is directly beside my air filter and there is a huge space under the air intake piping. Also, when you have cramed it in there....be sure to close the hood and make sure that it really fits ;P. Chances are you will not be able to use the huge bottle I did.
The bottle size will help you determine what kind of electrode you will be using and how much of it you need to obtain.
The location will determine how much wire and plastic hose you will need.

-updates-
-In my picture you may see the plastic container is bent a little where the plastic hose comes into it. I didnt just start bashing it. I aplied gradually increasing preassure until it bent to the shape I desired.

Step 3: Building Your Electrodes

I originaly intended to sodder my wire to the stainless steel. Go ahead and try if you like but it wont stick. So then I decided to drill and tap a hole in the pipe. I feel a clamp on connector would work better but I didn't have one. I did have a 1/4" bolt though. Wrap one wire around the bolt and screw it in tight. The bolt I used was very short and did not go through to the inside of the opposite side of the pipe. The screw in method I used probably saves more space than a clamp on.

-updates-
-Ok, so apparently regualr stainless steel does not work as an electrode. It lasted for 8 operational hours. People have said that 316L stainless will work better but I have my doubts. Instead I made them out of graphite. I give complete credit to hooloovoo33 for the suggestion in his instructable here( https://www.instructables.com/id/Separate-Hydrogen-and-Oxygen-from-Water-Through-El/ ). It is a very long and tedious prcoess the way I did it. I bought 2 jumbo sized kid's pencils(78 cents) and carved them down to the graphite core. I originally thought "I will just carve down one side and lift it out of the wood". WRONG. Pencil companys must use glue in their process becuase it dosnt come out easy. Even thin solitary pieces of wood clung to the graphite. What you can do is type in to google "buy graphite rod" and the machine shops that sell it will pop up(along with fishing poles). I found it as cheap as $5.86 for a single 1/2" by 10" rod, which would be enough when cut in half.
To connect the wire to the pencil lead I stripped back a long section of wire wrapped it around the lead twice and then twisted the wire back on itself and twisted like crazy with pliers. At this point I broke open the tube of sealant and dipped the end of the grapite with the wire in sealant. It gave me a great seal doing it that way rather than going over and over it with the tube's end. This was more a matter of frustration than technique. You need two conductors so do this twice.
After it dried I wrapped a big thick tye wrap around the top and bottom of each piece of graphite. Then wrap another tye wrap to hold them together. They should still be the width of a thick tye wrap apart. This is to allow the electrodes to be as close as possible without touching.

Step 4: Inserting Your Electrodes

I did this all wrong at first. I was cutting holes in the plastic with my knife. It is so much better to use a soddering iron to melt a hole in your grape juice bottle and then using a circular motion widden it it to fit your electrode. I had to seal over and over again on the knife cuts but the soddering holes sealed in one try.
Now you ask where do I put these holes at? I placed my electrodes on opposite sides of the container. I was thinking that the further apart they were the more water the current would pass through and create more HHO. I later read that having them closer together is actually more preferable as the electricity is "burned up" in the resistance rather than actually doing work on seperating the molecules. Please post if you think you know how it really works.(update: not necessary. Put them very close together)
However it works, it is very important to know that the electrodes are NOT continuous. That means they do not touch anywhere and they need to be secured so that while driving they do not run into each other. To secure my electrodes I soddered two holes next to it while it was in the container and fed some wire sheathing through to tie it down. It is likley this step will frustrate you unless you plan on big holes and more sealant. I would not recomend using any sort of metal(tie wire)because it will corrode inside the container. A tie wrap would probably be nice though.
The wire and connector on the electrode need to be on the outside of the container. Then you can begin the sealing. Generously apply sealant to anywhere water may leakout. Let it dry for 24 hours(so says the tube) then fill it with water. If it leaks drain it and put more sealant where it was leaking.
Advice from a big dummy: If your electrode is hollow like mine seal one or both ends. I will let you think why.(there's 24 hours wasted)

-updates-
-the graphite I used is much much smaller than the original electrodes but produces almost the same ammount of gas because they are so close. Holding the graphite down inside the container isn't as necessary because the tye wraps will always hold them a certain distance apart.

Step 5: From Container to Car

Now do the same for your plastic hosing as you did for the electrodes. Melt(or cut) a hole IN THE TOP and seal your hose into it. The hose needs to be long enough to reach the outlet coming from your air filter in your car. Melt(or cut) a hole in the hose leaving your air filter and seal in the hose coming from your HHO container. Ouch, I hated to do that step. Willful destruction of car parts. It helps a lot if you have an easily removable piece like I did.
I wanted to mention it somewhere and I thought the inserting your electrodes step was already to long, wrap some teflon tape around the teeth that hold the cap on. This will cause LESS HHO to leak out the top. I don't think its possible to completly stop hydrogen from leaking out of a container while allowing easy access to pour more water in.
The water you put in this container needs to have baking soda mixed in. I read that using salt will produce chlorine gas. That seems likley to me as salt is NA CL. Sodium and chloride on their own are deadly to you but combined they sure taste good. In other words don't use it.

-updates-
-I was going to use baking soda origianly but I later read that it lets out solutions that are corossive to aluminum. Chances are your car's engine is made out of it. Furthermore, the chlorine will be coming out of your tail pipe. Who breathes in car emissions that wants to live anyway? The chlorine released is small ammounts too.

Step 6: Giving Life to Your Monster

This is going to be the scetchiest part of my instructable. I am still searching on a good way to power it only when the car is on without having to drill through the fire wall, without having to connect some wires, or flip a switch everytime I get in the car. A wire from one electrode needs to go to negative and the other to positive of your car battery. I just ran the wires up to the top of the hood so I can connect them when I get to my car and tuck them in neatly the rest of the time.

-updates-
-Going through the firewall wasn't as bad as I thought. I just looked under the steering wheel and saw all the wires running through a rubber stopper. I rammed a fishtape through there and caught it inside the hood. I pulled 1 wire through and cut it. To turn it on simply connect that 1 wire back together. For now I am using wire nuts but it shouldn't be a problem to connect it to a button or switch. I took apart my dash because I was going to connect it to the rear defrost button(I have used it maybe twice in 3 years) but there was all kinds of circuit board wiring hooked up to it so I said forget it.

Step 7: Results

I have a 2003 Cavalier. The worst mpg I ever calculated was 28. I was purposfully flooring it every where I went just to see what the worst case scenario was. The best I have ever calculated was 45 while following closley behind truckers(they sure hate that). On average I fill my tank up and go 400 to 420 miles before my "pull over and get gas" light comes on. When my light comes on I am able to put 13 gallons in the tank.
400/13=about 31
The electrodes lasted for half a tank and I got 500 miles out of it. Assuming I got 200 miles out of the other half as usual: 500-200=300 300miles/6.5gallons=46mpg
46/31=1.48 thats a 48% increase in fuel economy!

-updates-
-With the graphite I got 510 miles out of a tank. 510/13=39
39/31=1.25 a 25% increase in fuel economy. I'm sure that if I increased the size of the electrode the mpg would increase. The pencil lead is just so small. I will update again.

P.S. My wife ran off with the camera to a florida vacation in the middle of my build. Thats why there are no pictures of the graphite electrodes. When she came back the camera wasn't working.