Basic instructions, words might not be the official crochet names.

For the loop, make a string 2 stiches wide, connect to itself at its start,
Cord, continue in 2 wide string. (If your mom does not know how to do 2 stitches wide string, start at the pocket end of the string, make a one stitch wide string long enough, and double on the way back with half stitches or whole stitches.)

When the string is long enough you start the pocket.
The first row is 2 stiches wide, because that is what your string is, now add two stitches each row till you have your pocket wide enough to your liking, now reduce the pocket by two stitches each row, till you have only two stiches left.

Now you make the release cord, two stiches wide should do.
You can also make it a single stitch wide and return to make it bigger.
If you like a release knot, make the string extra long, and tie a knot in it, or make extra stitches at the point where you want the knot. (I would tie a knot.) You can cut the extra length if it is too much.

If you add to the side, (both sides alternating,) you get a flat pocket, if you add in the middle you get a deeper pocket.

It does not matter much which stitch you use, but if you use a long stitch, you may want to add more stitches.
In my extra deep pocket I added extra depth by varying the hight of the stitches too, but that was overdoing it.

For size, take a leather pouch you like the size of, and try to copy that size.
You can also make a pouch by crochet and add cords of other string.
Success!

Willeke

PS,
Keep an eye on the shape of the pouch as you make it, it might be needed to add only one or three stitches each row.