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