You start with a jet that's a few sizes larger than the jet you have now. To find out what jet you have now, you'll have to take apart your carburetor and look at the little number stamped in the jet. If it's, say, a 56, then you could order jets like, 58, 60, 62, and maybe 64. Then when your jets come, start out with the biggest one you ordered. If it four strokes and fouls plugs, put in the next smallest jet you have. Keep doing that until your plug chops come out looking right.
As for a tutorial, there isn't one on upjetting specifically (that would be a good addition though, now that you mention it). However, there are videos linked in Fred's Guide about how to take your carburetor off the moped and clean it. That might help give you a better idea of how the parts come apart and go back together.