With an opamp.
This is how to make a square wave oscillator from an opamp with variable duty cycle (shown with 50%):
http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/UsefulCircuits/SqWaveGenerator.htm
Some general opamp information: http://www.opencircuits.com/Op_amp
also try this book, it's mostly single sided opamps, which is probably what you'd be using, and has a good collection of circuits:
http://focus.ti.com/lit/an/slod006b/slod006b.pdf
Alec