I do often hear students say they're not good with knots. Fortunately in tapestry weaving, you really only need to know a couple. One is a plain old square knot, sometimes done with an extra twist (surgeon's throw), and the other is the double half-hitch. Unless you are weaving on a floor loom and tying on a new warp, you can probably get way with only using these two knots for your whole tapestry weaving career.
I have had several online students tell me they are struggling with saggy edge warps this week, so I made this little video showing how to do a double half-hitch knot to end your warp. This knot is one that slides and you can tighten your warp up really nicely and it will stay.