Start small, and I mean very small. Half a leaf of a lettuce is a lot for a little bunny who’s never had it before. I’m talking a piece the size of your thumb or less. Feed that, and wait a whole day to check for poop problems. With no problems, go ahead and feed a slightly larger piece (two thumbs worth). Wait a whole day again, then feed a slightly larger piece again. And so on. Some rabbits are much more sensitive to greens than others, so I always take the slow is better approach and usually introduce a new green over the course of a week, and only one green at a time. If your rabbit experiences gas or mushy poops while introducing a new green, either stop the introduction completely, or go back to the last serving size of that green that didn’t give any issues, and feed that same size for several days in a row before increasing the size again. If your rabbit reacts really badly, it’s best to wait a week before trying again to let their stomach settled.
After a week of a good introduction with one green, you can then start with the next one using the same process. My method is a better safe than sorry method as some people’s rabbits seem to manage well just suddenly getting a half or whole leaf of something, but Bandit was always quite sensitive to new foods.