I've lied to you already. Not because this isn't a guide on how to use basic Omega-level industrial skills to make bank, but the specific example I'll be using uses moon materials (affectionately and hither on referred to as moon goo) instead of gas. The concept is really the same, though.
What if I told you that you're sitting on a gold mine of relatively passive ISK generation for only about two days of training? What about making nearly double that gold mine with two weeks of training? "Oh Cat, this is an obvious scam; just like ship insurance and that one time you said mining Kernite was the best ISK per hour," you might say. Well, strawman, listen up, because you don't even need a brain for this knowledge about to lay down on you.
There are three types of Reactions in EVE:
Biochemical, which is just fancy talk for drugs, reacts funny named gas and fuel blocks to make drugs.
Composite, which uses moon goo and fuel blocks to make "intermediate materials", and eventually composite materials used in tech 2 production.
Hybrid, which reacts together wormhole gas like C-60, some minerals, and fuel blocks to make hybrid polymers that eventually get combined with sleeper salvage and relic/data loot for tech 3 production.
and Alchemical, which lets you make raid consumables and flasks that we need to defeat the Lord of Flames, Ragnaros and complete [Thunderfury, Blessed Blade of the Windseeker] once and for all.
You can pick any of these paths for your ISK making journey, but just to highlight how lucrative they can be, I'll walk you through the easiest Composite example I have step by step what you need to do with only two days of training. Sorry Alpha-bros, this stuff is Omega only.
All you need to do in order to start is, technically, Reactions II. But then you would suck at it, so spend the two days and get Reactions III, and Mass Reactions IV.
If you want to make twice as much ISK, you'll need more reaction slots to perform jobs with. The plan above gets you 5 reaction job slots. If you want 10, you'll need to get Mass Reactions V, and Advanced Mass Reactions to IV.
Reactions has no prerequisite besides being on an Omega account, making it a great skill to train alts into. Spending 6 days to train 3 characters for 15 slots is much more efficient than training a single one to field 10.
Example: Carbon Polymers
I chose this example because it uses two moon materials you mine from an R4 moon, like the ones you find in High Sec or Wormhole space. You can use your own moon goo, or you can buy it from the market. If you're one of those people who insists that the ore you mine is free, then you'll love this - but even buying it will make you good money, which is the whole point. You don't need to do any collection work for this at all besides a little hauling, and we'll get to that. In this example I'll be using approximate ISK values of the materials at the time of writing. They can and will fluctuate. All the numbers were double-checked with Stella using ISK Per Hour, and they roughly check out.
Carbon Polymers are made with a blueprint that costs 10,000,000 ISK to buy off the market. The blueprint has unlimited runs, but can only be used for one slot. If you want to have 5 reaction lines running at the same time, you need 5 blueprints. Unlike typical industry blueprints, you don't need to research these things beforehand, so it's a one-time startup cost.
From the blueprint we can see that one run takes 3 hours, which gives us 240 runs a month. The cost of a single 3-hour run is 100 Hydrocarbons at ~1,800 ISK each, 100 Silicates at ~2,800 ISK each, and 5 Helium Fuel Blocks at ~16,000 ISK each. If we're using an Athanor with 10% tax, the total material cost per run comes out to about 540,000 ISK, or 129 Million ISK per month. That seems like a hefty amount of ISK to be paying, but the margins speak for themselves - each run that costs you 540,000 in materials is spitting out 200 Carbon Polymers, worth ~4,000 ISK each; a total value of 800K ISK, or ~270,000 ISK profit. For three hours of passive work. On a single slot. You can start to see how this scales pretty quickly.
If you're making 270,000 ISK profit per run, that's ~90K ISK per hour per slot. Scaling up to 5 slots (2 day training plan) over a month of time puts you at 324 Million ISK profit per 240 runs, with a material investment cost of about 648 Million. Oh, did I mention that a rigged Athanor does reactions 22% faster? And your level of Reaction skill gives you 4% speed per level? So you could be producing up to 42% quicker. It's more like 460 Million ISK profit per month.
"But but but," you say. "There must be a catch, it must be a real pain to haul, right?" Stop whining, straw man. It's actually not bad at all. For 240 runs, we're looking at 24,000 Hydrocarbons and Silicates at 0.05 m3 each, and 1,200 Helium Fuel Blocks at 5.0 m3 each. Bump that up by a max of 42% because of the rig bonuses and skills and that's only ~12,000 m3 per slot, so ~60,000 m3 for 5. If you find doing one trip in a Deep Space Transport per month a pain for these gains in the back pocket, I don't know what to tell you. The "catch", if you can call it that, is reactors can only be done in low sec, null sec, or wormhole space. You get three guesses where you can find a reactor in this corp, and the first two don't count.
"But but but what about the things you make, won't they be a lot biggerer?" Well, you got me there, strawman. Just kidding! Using all the same numbers, bonuses and all, it makes upwards of ~13,500 m3 per slot per month, or 67,500 m3 for 5. Not a big difference; materials in, products out. And if you really are allergic to a few hauler trips, you can keep the product (Carbon Polymer) in your reactor, not ever move it, and react it with other things, like Crystalline Alloy, to make the next level of materials, like Crystalline Carbonide. And then you can use those materials to produce t2 components, which use industry slots instead of reaction slots, so you can do them at the same time. The rabbit hole goes as deep as you want. But that hole is beyond the scope of this little primer guide.
Pick a product. Probably don't all pick this one mentioned above. Do some research. I linked the ISK Per Hour tool - learn to use it, and figure out an estimation of what you can get from different products. Want to keep that gas you find around Placid low sec or in wormholes and put it to use? React it. You already have it. Why keep it lying around your hangar - spend some time training, put in a nice long reaction job, and turn that gas into cash.
A real cat at a keyboard, and you're only able to read this because we live in the one universe where by sheer chance and coincidence, the cat is creating coherent sentences and playing games by hitting random keys.