Step 1: Scoop up a couple old bikes
Find two old bike frames. Check the Craig's list free section, your local bike kitchen, friend's garages, drainage ditches, bushes, or wherever else free bike frames are to be had.
Step 2: Cut back off of frame
Using a hack saw, or if you are luckier than me, a metal cutting band saw, cut the top tube and the down tube of your frames where they connect to the seat tube. This leaves the rear fork triangle thingy. Find somebody that needs the front of the frame for their other sweet project.
Step 3: Grind frame flush
Grind and brush the front of the seat tube so it's flush and clean for welding. The tube that the pedals go into needs to be cut down so the seat tube is flush all the way down. Grind and brush that too so there aren't any nasty burrs left over.
Step 4: Scavenge some angle iron
Find two length of angle iron that are no longer than the seat tube on either of your frames. The angle Iron should be large enough to fit around a sign post, about a two inch flange.