Because of its nature, its not really something you can make a 'guide' to. Just look at pictures, and decide how to do it, since there are many methods.
Basically you want two frames that are either exactly the same, or in the very least, have the same diameter head tube, and when you lay them on the ground and line up the headtubes, the bottom bracket of the top frame will sit square on the seat tube of the bottom bike. Then you just miter the seat tube of the bottom bike so it will fit the bottom bracket.
You need a tube that is the same diameter as the head tube, which you will cut to length and weld in between the two head tubes. I use a long piece of angle iron to clamp the two head tubes in, and the headtube extension. Then I C-clamp the mitered seat tube to the bottom bracket. I tack weld everything first, then start removing the jigs as I finish up the welds.
Then cut a gusset to go between the top tube of the bottom frame and the down tube of the top frame. They can be decorative, but they are a good idea, most broken tall bikes seem to break in that spot.
Then cut your fork's steerer tube just below the threads. Get a long tube to extend the steerer as long as you need it to go to reach all the way through the new head tube. Top tubes of additional frames sometimes are the right diameter. Weld the extension tube into the steerer tube, using that angle iron as a jig to line them up again.
I take the rear dropouts, cut them off, and flip them around and weld them back on so that they allow for adjustment along the same line as the seat stay, instead of the chain stay, that way you can run your chain directly from the top bottom bracket. Expect to have to bend the rear triangle around a bit to line up the chainline.
To solve brake clearance problems, I take a small piece of flat steel and weld it between the chainstays by the frame, close to where a kickstand would go, and then drill a hole in it for mounting the rear brake down low. That way your chain won't get all tangled up in the brake mechanism.
If you want to make the bike easier to ride, use a 24" fork and wheel in the front, and a 26" wheel in the back, that brings it forward a bit, and makes them less likely to wheelie.
Then I do nice things like grind down the welds, sand it smooth for painting, remove the rear triangle of the top frame and any unused cable guides and hangers. Paint the frame if you want, then build up the bike like any bike.
We use Sturmey Archer 3 speed rear hubs, and a BMX style one piece crank, typically. The shifter and brake cables are all run full-housing usually, with the housing carefully ziptied to the frame.
I like retro-goodness like shiny chrome rims, 12" high rise handlebars, a bell, and a cool bullet shaped headlight. I take pride in making my bikes as 'pretty pretty princess (as my friend calls it)' as I can, while still using only recycled parts. The only things I buy are cables and housing (since you need such long ones, sometimes you need tandem-specific cables), and sometimes brake pads and paint.
Good sources for parts are old department store mountain bikes (many of our tall bikes are made from Magna frames), old rusted out three speeds (that probably have usable or fixable SA hubs), and kids bikes. My tall bike's headset is actually from a 16" wheel pixie bike!
If you have any specific questions, let me know. Once you have two compatible frames (same headtube size, and bottom bracket lines up with seat tube, I prefer girls bike on top, but it doesn't matter), the rest will become obvious. Just make good welds, when in doubt, add a gusset, and have fun. Post pics when yur done.