All the advice so far is really good. I had my ileostomy in March last year and battled a long time with leaks and irritation, although this was partly due to chemo complications causing fistulas.
This is now a rare occurrence for me and usually only when I go a little too long without changing the bag! I therefore wanted to pass on a little more info that I gained through much trial and error.
I changed to a Hollister bag and use a little paste around the stoma where I have a crease before applying a Hollister ring. Before I remove the previous bag, I cut the hole in the new appliance. I don't go for pre-cut as I find my stoma changes size and shape. My stoma nurse said if using a ring, the hole in the bag should be slightly bigger than the stoma, resting on the ring.
Once the hole is cut, I put the new bag and the ring against the skin on my chest, under my clothes, with protective seals on of course. I find this warms the adhesive and gives a better stick.
Once I have removed the bag and cleaned around (by the way, I only use adhesive removal spray and then warm water), I dry the area and add the paste. I then remove the cover from the ring and stretch to fit. I also slightly flatten the outer edge of the ring. You can get a good snug fit with a ring far easier than with just a bag as you can see through it easily when applying. I press the ring until stuck and apply the bag, pressing it onto the ring and also firmly around the outer edge of the ring.
As a rule, I try to remain still for about ten minutes. I do mine lying on the sofa.
Also, if your output is very watery, this will make leaks much more likely and hard to get leaks under control. My stoma nurse stressed trying to thicken the output as the longer the bag etc. is on before being 'hit' by watery output, the better the stick and less likely the leaks. Hints to thicken, from my experience only of course, Imodium/loperamide - my stoma nurse said you don't need to restrict yourself to usual dosage but can increase, but only gradually, mashed bananas, products from white flour like white bread, cake, although not high fat, noodles, rice and maybe avoid too much fresh fruit and veg, adding slowly as I find the output with that lumpy but watery.
Sorry for such a long post, but it took me so long to learn to manage the leaks that I wanted to pass on as much detail as possible.
And finally, it will get easier with practice, so do not despair.