@RavenCat Check out a clip on Youtube entitled "How to end a wall hanging, a bookmark or a keychain" by Vlatkaknots. The explanation for this particular end starts at 07:21. Good luck!
@RavenCat I simply made backward-forward knots on the left side and forward-backward knots on the right side, each from the outside of the bookmark toward the center. First, take the two outer strings and knot them. Then take these two as one and knot them with the next string. Then take these three as one and knot them with the next string, and so on.