Yeah it is, that was the most involved mod on a car I've done, and I got it working with no problems using Robin's guide. Hardest part is cutting the plastic and removing some stubborn trim parts. But just take it slow and follow the guide, I found you need more force than feels comfortable for some panels but none broke.
As
@ch1z64 said, you want to cut the holes slightly smaller than the light, so that it clips in. On my first door I cut it to big, so just used a bit of superglue to hold it in place. (if your upgrading to LED's, don't glue till you've swapped the bulbs though...speaking from experience...)
Definitely worth the effort once done. I'd also recommend getting footwell lights if you don't have them. Also supplied by
@RobinA3 and even easier to fit than the puddle lights.