Need help composing an Isorhythmic piece of music.?

It's the opposite of what you suggest - it's writing melodic material over a recurring RHYTHMIC motif (which doesn't necessarily coincide with the melodic phrasing). You should look-up isorhythm on the internet before tackling this and try to look at some examples of isorhythmic pieces from the middle ages (eg by Guillaume de Machaut).