What seems to be the major problem?.. I am pretty old, hate bending down (big gut), and have arthritic fingers, but I don't seem to have a lot of trouble. I have the kind of coupler with a push button on the side you hold in with your thumb (yeah it hurts) and slide it on the splines.
Now some tricks: If you support the weight of the shaft with a bungee cord it is less tireing. Makes sure the coupler and the splines are well lubed up. I use Fluid Film. Inspect and make sure there are no burrs in the "lands and grooves" of the splines on either the PTO shaft coming out of the tractor or the coupler on the PTO shaft itself, if there are any, remove with a file. . If you have the pull back collar or the twist collar kind of coupler make sure before you get down and struggle with it, that its function of retracting the retaining balls is fully lubed up and working. Work on it on your bench not bent over out on the ground.
Make sure you can either turn the PTO shaft out of the tractor or the implement so that you can line up the splines. Makes sure when it does slip on the shaft, you shove it all the way forward, let go of the retainer (ring, collar, or pin) and pull backwards on the coupler to make sure it is retained. Don't want it slipping off and beating the he77 out of everything in sight, and then flying off in the weeds or over your head.
I used to have a lot of trouble, but when I started taking my own advice, and made sure things were clean, deburred, well lubed and supported, it is totally a non event now.