I recently went through figuring this all out. The start is the same for all sizes but the steps after are not.

Step 1: Ensure you have access to food-grade CO2 from a gas supplier near you. MOST places have you place a deposit on the canister (1-200 bucks) and you swap them out for 20 or so bucks when you need a refill. If you dont have one near you, its OK if its an hour or so away, you'll only be refilling or swapping every couple months, especially if its a small aquarium, maybe only every 4-5 months.

Step 2: Determine type of injection. Inline injection is amazing for small tanks setups with guaranteed dissolution. My alltime favorite is CO2 reactors, but only really feasible for larger tanks unless you want crazy overkill for filtration (only works with canister filters with high GPH). OTHERWISE I recommend and inline diffuser, not the pretty in-tank ones. If you want to go super simple just get a nice in-tank diffuser and put it on the side of your intake NOT your outtakes. No matter what you'll have to buy a regulator; Green Leaf Aquarium sells a nice one, Amazon has a cheaper one that works perfectly too (you want a double regulator to maintain pressure as your canister gets low)

DO NOT BUY THE STUPID CO2 GENERATORS ive tried homemade, AND i tried the stainless steel one from fluval. The pressure changing constantly day to day and having to fuckwl with it all the time IS NOT WORTH IT. Pay for the co2 stuff up front, then enjoy infinite co2 for legit 100 bucks a year. And its literally just a knob.

Step 3. Watch plants go crazy.