Here are the steps I used to build a tall bike. This one does require welding and some blood sweat and tears, but it is worth it.

I constantly hear "nice bike", or "how do you get up on that thing". It's not a bike for an introvert.

Step 1: Parts and Equipment Needed.

Start hitting garage sales and find yourself some equally sized tall bikes. I picked mine up for $5 - $10 each. Bring a magnet to make sure you are buying steel frames.
Avoid Chrome molly frames as it is wickedhard to weld.
Measure from the head tube to the seat post. The matching bike frames should be within 1/2 inch of each other. If they aren't close your top bike won't sit very well.

I recommend using a girls bike on the top, so when you dismount the bike, your foot doesn't get caught on the top bar.

Optionally, you can chop off the back triangle of the top bike. This assists with mounting.
If you leave the triangle on, you can add a third wheel, for looks.

Parts needed:

2 bikes of equal size. A girls bike on top, is preferable.
Extra long bike and gear cables.
1 black pipe for steering shaft (should match the inner diameter of your forks.)
1 smaller front tire, or else you'll have a UPD (UnPlanned Dismount).
1 smaller front forks to help with the center of gravity problem.
Chain link set (new or used)
Chain breaker (AKA chain link remover) ($5-$25)

Equipment needed:
Welder. I bought a cheap $100 110v arc welder.
Welding helmet, gloves, slag hammer and welding rods (I used e7014 on 45 amp setting)
Spray paint
Bike grease
Chain oil
Sanding attachment on portable drill. You have to grind the paint off the areas you are going to weld.

Optional Equipment:
Hacksaw, Sawzaw and/or chop saw.
Bench grinder, with steel brush attachment
Blow torch for sticker removal
Paint thinner for sticker removal
Glass beader for paint removal (if you're a perfectioninst)

Step 2: Cut and Weld the Front Fork Pieces.

Chop off the threaded portion of what used to be the forks to the top bike.
Drill four holes just below the threads.
Drill four holes into the fork tube just below the treads, on the bottom forks as well.
Insert the steel rod and weld it through the four holes.
Assemble the forks with bearings, nuts and all. Place all of the bike parts together and insert the rod through the top fork bracket.
The hard part here is estimating how much top rod to cut off. Mark where your top threaded piece needs to come out of the top fork bracket, estimate and mark your steel rod.
I made the mistake of not allowing for enough space to insert the handle bar assembly into the top of the threaded top fork assembly. Keep that in mind when making the top of the fork.
Cut the steel rod (preferably with a chop saw) at the correct length.
Weld the top of the steel rod to the top threaded piece. Make sure it is nice and straight.
Let it cool and then temporarily assemble the fork piece, while making sure the bottom of the bike sprocket housing for the top bike, lines up nicely with the seat post of the bottom bike.

Alternate design: My latest bike I created the drive shaft after welding the bike. I used a long rod to get the alignment. Having done both ways, I can't say which way is better.

Note: when welding, your frame will shrink a little, making the threads a little to long. This is hard to estimate for shrinkage.

Step 3: Weld the Seat Post to the Bottom of the Tall Bike.

Align all of your pieces nicely with a bar clamp.
Triple check that the bike is aligned from all different planes.
Weld the seat post to the bottom of the tall bike.

Alternate method: use 2x4's to clamp the bike together to get the main alignment just right.

Step 4: Add a Pipe Behind the Front Forks

I used a third BMX bike tube to create an added support beam just behind the front forks.
This takes some serious grinding to get the shape to match the connecting pipe.

Step 5: Finishing

I have built at least four bikes, which is why you see multiple frames. I thought this might help you design yours.
Remove all stickers and decals from the bike. The paint won't adhere to stickers as well.
I used a blow torch to heat up the stickers, until they bubbled. They then peel off easily.
Grind down any ugly welding joints. Don't grind too much as it may weaken the weld.
Clean the bike and get off any sticker residue with paint thinner or goo gone.
Spray paint it what ever colors you like. I went with tiger stripes on one and silver with splatter paint on the other.
Paint slowly and let it dry between coats. I used a paint pen to apply the black tiger stripes.
Either sand down the chain or buy two new ones. I used the wire attachment on my bench grinder and cleaned the chain, good as new.

Step 6: Assemble the Bike Parts

Attach the rear derailer (if you went with a 10 speed)
Put on the wheels. I recommend a smaller wheel in the front so you don't pop wheelies unexpectedly.
Put on the handle bars. I took girl handle bars, chopped off the top bar which gives it that chopper look.
If in doubt, take pictures of the pieces as you take them apart.
I also recommend the atomic bike builders book. I got several good tips out of there.

Put the sprocket assembly back together onto the top bike frame, using plenty of good grease. Re-attach the left pedal. Don't forget that left pedals, screw in in reverse.
Attach the seat.

I spent a lot of time determining the length of the chain. I ended up adding more than 6" of chain to the first chain. Remember it's easier to shorten a chain than it is to lengthen it.
I recommend buying a master chain link set. It makes the chain much easier to put together and take apart. Once your pretty sure the length of the chain is correct, assemble it through the derailer and through the proper parts of the frame and sprockets.

Attach the rear brakes. Run your cable up to the handle bars.
It's probably easier to just buy new longer brakes cables. I had to mount the brake handle half way down the handle bars.
Attach some new handle bar grips.
Nobody recommends front brakes for safety reasons.
Triple check that you tightened all of your bolts.

On my latest triple tall bike, I added a bar on the font fork and mounted front brakes. They seem to work well.

Step 7: Final Notes

Wear a helmet.

Mount from a bumper of a van or learn to free mount.
I free mount by running along side of it, stepping my foot on the bottom tube and sliding onto the seat.

My daughter just has me hold the bike and push her forward. She learned how to ride this in about 10 minutes.

Dismounting involves slowing, throwing your leg over the bar (riding side saddle) and then hopping down. Sometimes I step down onto the bottom tube.

Enjoy building your tall bike. Consider forming a local tall bike club. Google for "tall bike" or "freak bike" and you will get many ideas for different bike configurations.

Cheers.