I’ve been living in wormhole space for a while now so here’s how to do it:
If you do live in a wormhole, you can trigger the sites, wait for rats to spawn, kill them, then harvest the entire cloud. If you don’t live in a wormhole, grab your scanner and grab a venture.
Scan down gas sites but don’t warp to them yet. Once they are warped to (uncloaked) rats will spawn after ~13 minutes.
Instrumental, Vital, Vast are the only ones I consider worth my time. Once you’ve scanned them down go switch to your venture. If this is going to be a main source of income then its worth training into T2 harvesters (50% higher yield). Otherwise, stick with gas harvesting II. Slap a MWD on it then warp to the site, set a timer for 13 minutes, and MWD toward one of the two gas clouds and start harvesting.
Rats/npcs don’t spawn in those sites until sometime after 13 minutes; once that timer goes off you should actually pay attention and warp off once they spawn. You could spend those 13 min spamming dscan to stay safe but ventures are cheap enough its not worth the effort.
Currently, each reaction will output you profit so its worth doing if you have the skills.