If your planning of making then follow the many examples in the threads here.
Id recommend you go for a design that avoids using a centre upright post, and instead go for a design that has the upright support from the sides meaning their is nothing in the way of the pedals.
Making the frame is quite easy, getting the pedal, seat and wheel positions to be comfortable and more importantly designed that you can allow for adjustments is the area you should concentrate on
