It's an awesome skill to have, building your own wheels and it most definitely is very common for freestyle bmx to run 48 spoke 4x wheels.
You can definitely look at some of the tutorials online (bmxtrix.com has the same as G-Sports) and you can just ask the bmx shop you buy from what length you need.
Determining length is something THEY need to get right, not you. So, if you give the shop your rim brand, your hub brand and how many crosses you want your wheels (0, 1, 2, 3, 4) then they should give you the correct spokes.
When I built my first wheel, no Internet, so it took me about 12 hours or more. Now it takes me just a couple of hours to do a good job. Never had a truing stand and I most likely never will. But, a spoke wrench is an absolute must. They are inexpensive and will take care of your building needs.
Most of all - Give it time and be prepared for it to take all day. Don't just do it the day before a big event if that is your only wheelset. Make sure you have the time for it, and make sure you can live with having to redo it several times if you don't follow the instructions precisely.
I have never used Loctite on spokes, and never plan to. Rims are constnatly being tweaked and it has very little to do with spokes loosening. The 4x pattern will also help to hold everything in place. 3x isn't bad - but neither is 2x or 1x. It just isn't as stable as a 4x pattern overall.