I do it the simple way. For the single wire, heat the stripped end of the wire and allow some solder to flow onto it. I do the same to the lug. Then rest the wire against the lug and touch the iron to the wire so the solder melts again, and flows into the solder on the lug which will have started to melt as well. Hold steady while it sets (blow a little onto the joint to speed it up). For the pair of wires, I twist the wires together first, flow solder onto the twisted pair, then do the same as for the single wire. Sometimes you will need a little more heat now that the mass of solder is greater on the pair. Sometimes there are three wires (on the ground side) and threading them through the hole just isn't possible. Probably no reason not to thread the single (or pair if they fit) in the hole before final soldering, I've seen that done regularly enough.

Doing a pair or three "one at a time" won't work as the first will just come away as you heat the wire/lug for the second one etc.

If anyone has a better way be glad to hear as I always wonder if I'm doing it right!  :)