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.

Press well.