Your trousers or shorts need to be:
2 1/2 times the length of your finished turn up plus a double hem longer than your desired length.
Step 1: Hem the Bottom
Pin up a double hem. Sew down & press.
Turn up 2 1/2 times your turn up depth.
Step 2: Fold & Sew
Fold the hem up half the depth of the turn up & press.
Hold where you want the top of the turn up & fold the trouser in by half the depth of the turn up & press.
Use a large zig zag invisible prick stitch to hold both the hem & the turn up.