It can be difficult, for various reasons. It's harder for men as a rule because we're slightly differently proportioned and often less flexible than ladies. Some ladies, of course still find it difficult, so plenty of practice would be a good idea.
Most of the advice here is fine, but I should point out that you need to have a suspender belt with METAL clips on it, not plastic, and it's easier if your stockings are plenty big enough - i used to have terrible trouble with that but nowadays women are taller and it's easier to fing stockings that are long enough. I find that with metal clips, it's not too hard to reach round to do the back ones up.
I do think you should have your pants on first - it looks far better even though it's a little less convenient. Unless you have a very small/weak bladder you should be able to last out a fair while without having to undo everything, and if the stockings are long enough and the clips are metal, it shouldn't take long to undo them and then do them up again when the call comes! If you're wearing stockings specifically for bed, it's probably better not to bother with pants ... .
Some people find that even with all this right, the stockings tend to slide down their legs. You can try putting baby lotion on your legs first, which helps them stay up, or you can use hold-ups instead of ordinary stockings, which also helps them stay up. In fact, if your clips will go round the tops of hold-ups OK, it's the easiest way to keep everything looking absolutely perfect. Unfortunately, not all combinations of suspender belt and hold-ups will work in this respect.