There are fly boxes that hold the flies on hook threaders in the box, much like the threader tool people who sew use to thread needles. You line up several of the same fly on each threader at home, where you can use a magnifier and rest your hand on something to steady it. Then changing flies on the water becomes very easy. You just poke your tippet through the large eye at the top of the threader and pull the fly off.

Try the Weaver's Knot shown in the FAOL knots section for your fly to tippet knot. It requires very little in the way of small motor skills and can be tied with very shaky fingers. It is even easier if you temporarily attach a little soft weight to the hook while you are tying the knot.

Good luck!