Step 2: Bike layout and design
This took quite a bit of time and experimentation.
I started with a large piece of cardboard. You can get a bike box or two from your local bike shop. Mine was a weed trimmer box from my local lawn tractor dealer.
I cut up and used various pieces from several bikes to get a sense of how the bike might be configured. I started with some small frames I got for free from (my local freecycle.org). I ended up buying a larger framed bike for $40 from Craiglist to cut up.
In any event, do the layout and figure out what frame piece you can reuse and assemble into your new frame and what pieces might need to be fabricated from conduit or other steel tubing you can find.
I used the primary donor bike frame along with (oval) pieces from a bmx bike to form the horizontal bottom tube and 1 1/2" rigid galvanized electrical conduit for the new extended top tube. I ended up cutting the seat tube and inserting it through a hole in the conduit then welding the seat tube back together. I welded it together with an outer strap in order not to reduce the inside dimension required for the seatpost.
Note the drawing showing the wheel locations, head tube/fork angle, bottom bracket height, etc. Also critical here was the measurement for the swing of the crank and the rearmost dimension of my (shoed) heel in order to clear the rear (stationary) swingarm frame.
I returned to this layout drawing many, many times. Take you time and work things out here where changes are easy to make.