I don't think there is an easy way. I would start by cutting two blocks of wood from a 2x4 using a miter saw. Lay your wheel flat on your work bench, sprocket side down. Precision cut two blocks that will just fit between the rim and the work bench. 90 degrees on the rim away from the broken spoke You may have to cut them several times to get them just right. This will keep the exact offset true when you go to re-tighten your spokes.
Next, when you loosen all the spokes, try and use the exact number of turns on each. If you turn your wrench 90 degrees, 6 times, do every spoke exactly 90 degrees, 6 times. If you need 3 more times to get the new spoke in, do all spokes 3 more. ( I made these numbers up as an example, each wheel is different, just count the same number of turns for whatever it takes until the wheel will offset enough to get the spoke started.)
To tighten it back up, use the exact same number of turns of the wrench you used before. In my example that would be 9 turns of the wrench (yours might be as many as 25) you may want to do it in stages like go around the rim with 3 wrench turns then 3 more then 3 last turns =9 turns total. If it were 25 wrench turns, do 5 turns on each 5 times.
Keep your wood blocks in place as you tighten so that the offset stays the same and allows the spokes to pull up without any weight on them and check to see that the rim is just barely touching the blocks when the spokes are tight. It will tell you quick if you miscounted your turns on the spokes .
If you do this correctly, you should be very close to true. By breaking a spoke the rim may or may not be slightly tweeked. At this point most motorcycle mechanics could get it perfect in 5 min.
Its not as hard as it sounds, just take your time. Use a piece of masking tape to mark the spoke you start with.
I'm not an expert but have done this several times, someone else may have a better idea, please chime in.....Hope this helps....Dave
P.S. If you cannot get the new spoke half to thread on this way then it is the wrong spoke!