This is a really rewarding project. Once you have mastered inserting a zip fly the rest is a doddle!
(I don't mention it all the time, but, as a general rule: press everything as you go along!)
Step 1: Find a Good Pattern
This one is from the Great British Sewing Bee book 'Sew Your Own Wardrobe'. I've shortened it for shorts.
Measure yourself accurately & choose the correct pattern size.