Lubrication is the number one piece of advice given for new
GS users and with good reason. If the
GS is sticking to everything in sight, it becomes a nightmare to work with. As for
how to lubricate, you've already seen that there are different options.
Water works without needing cleanup, but is is limited in its lubricating abilities. Vaseline or oil works better and won't evaporate, but requires more cleanup. Personally, I use the middle ground - a water-based personal lubricant (something like KY jelly) thinned with water. It behaves more or less like water but evaporates somewhat slower and lubricates better. Cleanup is simply a matter of rinsing off the residue - no scrubbing or soap needed (for small jobs, a wet brush suffices).
Most of the other advice applies more to sculpting than (ideally) simple gap-filling. Just to paraphrase a less than exhaustive list: lubricate (it bears repeating), work in layers, use the varying firmness during the curing process to your advantage, a variety of sculpting tools is helpful but one can get by with nothing but a finger and a damp toothpick, and practice, practice, practice!
There are hoards of tutorials out there, but most that I can recall are for specific sculpting tasks (how to
GS robes, ropes, etc.). Identify the task at hand, then make friends with Google.