To shorten trousers, a professional would need tailor's chalk. It is practical because you can mark the clothing visibly, but it does not stay on the clothing like pens, for example. The chalk can simply be wiped off again or removed with the next wash. Well, since I am not a professional, I naturally don't have anything like that at home, but there is a cheaper alternative to still be able to mark your clothing well:
Using soap as tailor's chalk
If you don't have tailor's chalk at home, you can use a piece of white soap. It leaves marks just as well and can be wonderfully removed with the next wash.
Read also: Sewing for beginners: what do you need?
Photo by Sabine, meinhaushalt.at