The best frame to begin with is a ladies MTB frame. A smaller frame than you would normally ride is best. if you are under 5'6", a 24" wheeled bike is best. The construction photos show how we did a 20" wheeled one. You will need another scrap bike for parts.
Once you have stripped the frame of parts, Cut the down tube nea to the bottom bracket, and the 'top tube' nea to the headset.