I'm no expert, but I have done this twice now, so for what it's worth....
0. Buy a blank backplate to fit your spindle. (Yeah, you can machine from scratch before this step, but for your first chuck,... don't.)
1. For a threaded spindle, the first goal is to get the backplate mounting flat against the shoulder of your spindle. That way, every time you mount it, it will end up in the same place.
Mount the backplate "backwards" on the spindle; that is with the neck facing outwards towards the tailstock. Face off the surface that will meet the shoulder, and you may have to bore out a small depth inside to get it to fit.
2. The second goal is to get the front face of the backplate perpendicular to the spindle axis. Turn the backplate around and mount it in its usual orientation. Face it off.
3. Measure the recess (diameter and depth) on the back of your chuck. Also note where the flat surface on the chuck back is (the surface that you want your backplate to sit on), and where the mounting holes are.
The usual situation is that the flat surface and mounting holes are outside (at larger diameter than) the recess. The next goal is to make a protruding boss (think of it as a very wide, very short peg) on which the chuck will sit.
The boss should be a tiny bit shorter than the depth of the recess (so the mating surface meets) but the diameter of the boss needs to be as accurate as you can get it. Sneak up on it. Take your time. If you measured the recess diameter with the usual set of calipers, don't trust that measurement too much. Start taking a thou at a time well outside that diameter. Try the chuck after every little cut.
4. Pat yourself on the back; it's easy from here on. The next goal is to mark the mounting holes. Take the backplate off the lathe and put it on the bench. Fit the chuck on top of it. Mark the mounting holes using a transfer punch through the chuck holes.
5. Drill and thread the mounting holes. Good practice here helps. Good drill-press setup, and a tapping fixture will make sure you get nice perpendicular holes and threads. (This matters. (Ask me how I know. I have an extra set of &@%#$-ed up mounting holes on the first backplate I made.) The bolts pass a long way through the chuck, and there is very little tolerance for the mating thread to be out of perpendicular.)
6. Bolt the chuck on.
7. Open beverage of your choice.