Templates are the key. First someone has to measure the hem length up from the floor and pin the hem in place.
Decide what width of hem is needed, then cut a template out of lightweight cardboard. I use cardboard from the back of note pads or packaging. Use a paper cutter to cut the templates.
Having multiple sizes is useful to have around for reuse. In sizes such as 1/2-inch, 1 inch, 1-1/4inch, 2 inch, 2-1/4inch with each labeled in a non-visual method of your choice.
I like to use office clips to hold the cardboard even with the bottom fold of the hem. If office clips aren’t available then use large paper clips.
Next cut the excess fabric away. To do this, place the fabric to be cut between the scissor blades. Line the bottom scissor blade which is under the fabric against the top edge of the cardboard. Do this by feeling the fabric fold over the blade. Be careful not to catch the fabric under the bottom blade you don’t want to cut. Then cut.
I will often use appliqué scissors or pelican blade scissors to make it easier to not clip the fabric on the underside of the bottom scissor blade.
Once the excess is trimmed away the next step is to sew the hem. This is described in my Machine Sewing book.
Sample or purchase Needle Arts with Vision Loss: How to Enjoy Machine Sewing Without Sight: https://www.smashwords.com/books/view/530511 or https://store.bookbaby.com/book/Needle-Arts-With-Vision-Loss-How-to-Enjoy-Machine-Sewing-Without-Sight-POD
Enjoy trimming your hems without unintended cuts.