Wallpapering on boards, what do I need to do? (Pic)

[PlumFairy2014]

I have started taking woodchip paper of this wall, underneath is boarded (looks like thick cardboard to me, don't know it's real name).
Once all the chipped paper is off, how to I prepare the walls for wallpaper? Parts are a bit uneven where there are nail, some kind of filler needed?
Any help/advice greatly appreciated!!!!

Please help, shameless bumping.

That could be plasterboard and a shoddy job. How old is the house? If there's any possibility it could be asbestos sheet, ask a builder to identify it - not a cowboy who would rip you off.

Try to avoid damaging the surface.

Get all the woodchip off, then ask a builder for a quote for a skim finish, this is a smooth fresh plaster layer which is usually put over fresh plasterboard.

As the previous people skipped that stage, and if you want to, fill any gaps with Tetrion or Pollyfilla, smooth off and sand down. Make sure the platerboard is dry, then seal with a thin coat of size.

Either re-paper, or use lining paper, then emulsion paint over that.

Good Luck, DIY always involves loads more stages then you think.

Thanks so much!
The plasterboard isn't even cut straight so I reckon we'll have it re-plastered and skimmed (as we did with the ceiling and other wall....)