Sewing Machine/ Needle if you're hand sewing
Step 1. Get yourself a pair of trousers that you want to turn into shorts.
Step 2. Cut the legs to just below the length you want them to be. You need to leave enough room for a hem.
Step 3. If the trousers are too big the put them on and measure how much you need to take in each side. Then take them off and pin all the way down each leg.
Step 4. Sew along the pinned line (remembering to take out the pins of course!)
Step 5. Turn up the bottoms of the shorts twice so that you don't have a raw edge showing and sew that. It's always good to keep measuring as you take them up too so that each leg is the same length.
Step 6. You're done! That's basically it! I've also changed the button to a nicer one too. I hope this tutorial was helpful, let me know if you try it! Enjoy :)