If you've decided you might like to race cyclo-cross, having some barriers to practice with is a really good idea. Having some that you can throw into a backpack is an even better idea. This instrucable should help you to achieve this goal.
Step 1: Gather Some Supplies
The dimensions and list of stuff for this project comes from here, I'm just going to add some details and one improvement.
To make four barriers you'll need:
8 x 1/2" Side Outlet 90 degree connectors (the 3 way thing in the photo
If the side outlet is threaded (mine are) then you'll also need 8 threaded 1/2" male adapters
12 x 1/2" male to male connectors
OR 8 x 1/2" male to male and 8 1/2" T-connectors, this will make a slightly sturdier barrier
1/2" PVC tubing:
16 x 22" for the center horizontals
16 x 19" for the outside legs (make these longer or shorter depending on what height you want, 19" make a barrier about 14.5" tall)
4 x 12" legs for the center (optional, see later steps)
A pipe cutter or hacksaw if your hardware doesn't cut the pipe.
I got all of this at a big chain hardware store and decided to cut the tube myself so I also bought a cheap pipe cutter. Also consider getting a few extra connectors, they'll often break before the tube does when someone stands on, or crashed into the barrier.
If you don't want or need four barriers the basic run down per barrier is:
2 x side outlets
4 x 19" legs
2 x 22" horizontals
3 male to male, or 2 male to male and 2 T's
Step 2: Tidy Your Desk Before Taking Photo's (optional)
I went with leaving everything messy, it up to you really.
Step 3: Measure, and Cut the PVC Tube
If you didn't get the store to cut the PVC you'll get to do this now. Firstly start playing some music (I chose the new Arcade Fire album, which is excellent btw), this steps takes a while.
I made a simple jig on my desk so I could measure, mark and cut fairly quickly. Grab a piece of 90 degree angle aluminum and clamp it to a desk 22" from the end. Every work shop should have a length of this aluminum, it's a great straight edge, can mark long straight lines and can be clamped in all sort of useful ways.
So measure, then cut, then measure again. Do this until you have sixteen 22" pieces. Count as you go, I cut too many. Then move the aluminum so that you're measuring 19" and start again. At some point you might need to take a break.
The cutting tool in the picture makes a nice clean cut, but is kind of slow and gave me a blisters from spinning the tube. A hacksaw would work too, just try and cut kind of straight. You'll also need to clean up the ends a bit so that they slide into, and out of the connectors. They sell a fancy tool for this, I used some sandpaper.
Step 4: Make the Outside Legs
To make one out side leg grab one three way connector plus the male threaded adapter. Screw the male adapter into the three way, this will be where the horizontal beam comes out. Stick two 19" legs into the other two holes.
Step 5: Add the Horizontal Sections
Grab four 22" sections, and 4 male to male connectors, or if you want to make the barrier a bit more sturdy grab 3 male to male and one T connector. Put one 22" piece into the outside leg, then add a male to male, then another 22" piece until you have four, then add the other outside leg.
If you went with the sturdy version then make the center male to male a T connector, then add the 12" section with another T on the bottom. This center piece helps stop the whole lot sagging and ending up low in the middle.
If all that sounds confusing have a look at the pictures.
Step 6: Go Out and Practice
Mark the horizontal sections with tape or a sharpie, you'll be able to put the barrier together much quicker this way. Throw enough barriers in a pack pack, ride to you local park, assemble, and practice. People will look at you strangely. This is fine, you've taken up Cyclo-cross as a pastime so you probably deserve it!