Measure how short your trousers should be and mark it on the pipe using the erasable pen (you can just find these at office supplies in different shops). Now draw a straight line with your ruler, 1 cm above the marked spot.

Fold the leg outwards and place the hem of the trousers against the drawn line, right sides together. Secure with pins if necessary and iron everything well.

Now sew right into the seam of the hem. It's best done with the sewing machine, but you can certainly do this by hand too.

Then fold the hem down and the excess fabric inwards. If this is not too much, you don't need to cut this off.

Now iron the hem well flat and the fabric well inwards.