I finally got around last week to repurposing a pair of old suit trousers I fished out of the bag going to the charity shop. I'm not sure if this is a good thing or not, but it turns out my boyfriend is only a little bigger than me round the waist so not much needed to be done to make them fit.
I forgot to take a 'before' photo I'm afraid, or any during the process, as I kind of winged it, but I'll give you the gist of what I did:
I used this tutorial to help with taking them in at the waist, although I found to difficult/painful(!) to pin the excess whilst wearing them. Putting them on inside out to check though is a great idea.
I cut them off just above the knee, and rolled up the ends rather than hemming them. Then I used the excess fabric to make 1.5/2" strips for braces. I folded them like bias binding, folding the raw edges to the middle and then the whole strip in half lengthwise so all raw edges are enclosed. Then I stitched as close to the folded edge as possible. I then used a contrasting white thread to do a decorative scallop stich down each side.
Lastly I attached a strip either side of the centre of the shorts, crossed them over at the back and then attached them to the inside of the shorts.
I do look rather like a lumberjack with this shirt on as well.... but that's okay... right?