Put on the old pair of trousers and use some masking tape to mark a point just below your knees where you want the pirate's trousers to come down to.

Take the trousers off and cut the bottom of both trouser legs off at the tape marks. Then cut zigzags at the bottom to make the trousers look more ragged. You could also cut out some fabric patches - why not use some scraps of the material you made the top with? - and sew them on with big stitches.

Now you have an designer pair of pirate's trousers!