The key is to get the inner star washer and nut in the right position first. You will know that they are properly located because just the right amount of jack will be sticking out of the tail (enough for the the outer nut, washer and cap to fit without excess threading showing). Most endpin jacks have a very small hole through the sides laterally that you can use to stabilize the jack and keep it from spinning when you tighten the outer nut. Just thread the nut on far enough that it passes the lateral holes, and then use something thin like an allen wrench put through the holes to stabilize the jack and keep it from spinning while you tighten the outer nut. Since the inner nut is already in the right place, all you have to do is tighten the outer one and everything is good. If the jack spins at all, it will thread backwards through the inner nut essentially tightening it, which will mess everything up, so make sure it doesn't move.
Once you have the nut on, just thread on the endpin cap and you should be ready let it rip! Just make sure you double check everything (especially your solder joints) because its no fun to have to take the whole thing apart and start over.
As far as getting the jack into the hole through the block and tail of the instrument, just get a really cheap guitar cable and cut it in half (leaving about 3 feet of cable with a 1/4 inch jack on one end). Use one that has a rubbery encasement where the jack meets the wire (not a steel one, that's why I say cheap). Take a razor blade and shave down the rubber encasement so that its as thin as it can be without exposing bear wires (you want it thin enough that you can thread the nut and washer over the 1/4 tip completely). Push that end through the endpin hole into the body of the instrument and let it come all the way forward and out of the soundhole in the front. At this point (you should have the jack also coming out of this soundhole too) plug the cable into the jack, and use the other end of the 1/2 cable to pull the jack back into the soundhole and all the way back into the hole you have made for it in the end block and tail. You will have to do this a few times to get the inner nut and star washer in the right place (just try moving it around to different point on the threading till you get the right amount sticking out the back side). You can then thread the outer nut and washer down the cut side of the 1/2 cable, over your razor bladed thin rubber encasement at the jack end of the cable that is still plugged into the jack, and directly onto the threads of the jack. Tighten that thing down past the lateral holes, stick your allen wrench through the holes to stabilize, and tighten it up completely.
Make sure to tighten the nut enough to compress the starwasher. This will create back pressure against the jack that pulls the nuts tight on the threading and makes for a much more reliable hold (less likely to come loose over time).
This is really hard to explain, but I think its the best way to deal with endpin jacks, and certainly the quickest. Let me know if you want me to clarify any of the steps, I'm not sure if I explained well, but that's my best shot! Good luck, you can do it!
