In the UK and some parts of the US plaster was used as a finish on the walls of many homes.
Over the years ,the plaster becomes vunerable to cracks and general degradation of appearance.

This is when people start covering the walls with wallpaper.

This limits what can be done in home and it is a fallacy that the walls cannot be painted to lower maintainance costs.

Here's how to do it.

Firstly strip the walls of all paper,yes even the little bits you think won't matter,they do.

After you've done that was the walls down and use a 6" scraper to against the plaster to remove any remaining adhesive.This step is very important.

Allow to dry for a day.

Now if any areas of plaster are damaged get them repaired professionally or do it yourself if you know how.

Ok on the next step I use filler called EASIFILL45 it's made by Gyproc in the UK,it's an easy to sand filler for joint taping.
There will be a similar product where you stay without doubt,
Look up the gyproc site for your equivalent product.

This filler is like chalk to sand,hence it's name easi fill.

Now what you do is make up a half bucket (inc wqter) and get a trowel and hawk (for easier handling).
What you want to do is put a THIN coat over the entire wall,miss no area,cover the entire wall.
If you have deeper holes double back on them after you've coated all the walls.

Allow to dry and repeat the process,yes completely recoat all the walls missing no area.

Now you'll realise why you MUST put a tight finish on,in other words don't build up the filler but trowel most of it off.

Next use a pole sander prefferably one with a vacuum attachment and sand the walls down thoroughly.

Once done make sure the walls are perfectly smooth,and then check again,the more work you put in here the more the results will impress.
Vacuum the room.

Next apply a thin coat of PVA mixed4-1 and roller it on the walls,remembering to cut in as well.
Then after about 1 HOUR hang a 1000 grade lining paper on the walls(I will write a guide about this if there isn't one already).
Finish the room and leave to dry.

Now the walls will be looking good if you've done this right.
Obviously at this stage you'd paint you ceiling to a finish,2 coats of your chosen colour.

Done that ,great ,next first coat your walls with your chosen colour and allow to dry.
Then fill the seems in the lining paper,sand down once dry and touch up with paint.

Alow to dry and 2nd coat walls.
You're done,you can now change the colour of your walls easily from now on and no more expensive,stripping,repapering etc.
Enjoy.