Painting over wallpaper is generally a bad idea. It can cause so many problems, it is worth it to just make the effort and remove it before doing any other work on your wall.
If you really can’t be arsed however, and insist on just slapping the coat over the paper, try to follow these simple guidelines:
1. Check how many layers of wallpaper are on the wall. If it’s more than one, please reconsider removing them. They nearly impossible to paint properly!
2. If you still want to keep the wallpaper, then make sure it is firmly bonded to the wall. If there are several layers, make sure the bottom layer is firmly bonded to the wall, the next layer firmly bonded to the first layer and so on. Otherwise, they’re gonna peel and it’s goodbye paint job for you.
3. If the wallpaper has an embossed finish and you will still paint over it, be ready to have wall paint with bumps - fucking ugly as hell!
4. Before painting over the paper, coat the entire surface first with pigmented shellac sealer. That will give you a decent surface to paint on.
If you do, however, decide to listen to reason and remove the pasted paper from your interior, here’s a few tips to make it faster and cleaner:
1. Use specialty chemicals for paper removal. You can buy these from regular hardware stores. Most of them can be applied via brushing on the edges of peeled paper, allowing them to detach with little remains.
2. Borrow a wallpaper steamer - this device will heat the bonding materials and allow it to loosen much faster.
A last solution is always to just remove you wall entirely and put up a new one. That is highly recommended.