You need a pair of ostomy wafer cutting scissors that have curved blades and a blunt nose to avoid poking holes into your bag. You can get these from your ostomy product supplier. If you don't have one yet, overnight a pair from Medical Monks, credit card only.
After you have showered and dried the skin well (free of any lubricants, oils, etc.), you first cut a small hole and see how it fits around your stoma, then cut around a little larger and so forth. Some people have oval stomas, so a hole cutter won't work. Plus, with scissors, one can trim a bit here or there as needed.
You want the hole just a hair or two bigger than the stoma because it protrudes into the wafer as it discharges matter sometimes and thus expands a tiny bit. You don't want it getting stuck in the wafer hole or the sides of the wafer scraping the stoma, which could cause bleeding or even a granuloma.
There usually is a stoma measuring guide in your box of ostomy bags. So once you have a nicely test-fitting wafer, what you want to do is use a marker to touch the top edge of the unpeeled wafer and your skin, making reference points to line up later on. Some people's stomas change over time, so you need to change with it, and that's why a hole cutter isn't always the best choice, but it can get things started, then finish with scissors.
If you put the wafer on misaligned (and/or paste over the stoma), it can cause a clog or the stoma pushing the wafer off, causing a leak.
So once you have the wafer hole, you push up and in the sharp edges or burrs around the hole. If cut large enough and not misaligned when applying, the stoma shouldn't be touching it, only the ring or paste.
Next, you apply your ring, enlarging it to fit the stoma and pressing it down to squeeze it closer to the stoma. Make sure you have it right side up.
If you're using paste, you squeeze from the tube with the nozzle about 1/4" - 1/3" high off the skin and go around the stoma like that twice to cover the flat part of the wafer contacting the skin. The width is important as it holds better if it's wider, just like a ring in size. If you have unusual dips or folds, you add more on top but watch out that when applying the wafer, the paste does not go past the edge of the wafer hole when pressing down later, just barely see it stuck out a little. This can be tricky, but it does protect the edge of the wafer from irritation on the stoma as it moves up and down. If you cover the stoma, it can cause clogs and leaks.
I'm saying all this because as a newbie myself once, I thought my stoma was round, matching the round guide marks on the wafers. Well, it's not, it's actually oval, and I went for many months suffering, cutting round holes and having leaks because when pressing down, there was nothing pressing the paste into the skin where the gaps were. I finally wised up and started test fitting my wafers beforehand.
Another piece of wrong information I got from stoma nurses was to apply the paste to the wafer, then apply it. Boy, that was a huge mistake because gaps were forming and leaking, plus the wafer was going on misaligned. Plus, sometimes the paste (or ring) won't adhere to the skin because it's too damp or oily. So by putting the adhesive on first, I can check that and redo before wasting a wafer, etc.
With very clean, dry, and oil-free skin, it should easily stick, but you don't know that unless you see it and test before applying the wafer.
Good luck 🍀