Wallpapering textured walls requires a bit of effort in order for it to come out looking good.
Wallpaper will magnify any texture, bumps or flaws in your wall so you will need to make the wall smooth before
applying any paper. There are basically 2 ways to accomplish this - with drywall compound or with liner paper.
Using drywall joint compound to provide a smooth surface before wallpapering textured walls is the most time consuming method.
Sand the walls first lightly, then apply a thin coat of joint compound over the wall to hide the texture.
Let it dry and sand again. You may need more than one coat to make the wall smooth. Once this is accomplished you should seal the
surface with an oil based sealer after making sure all the dust is removed.
Another way of smoothing out a wall is to apply wallpaper liner paper.
This is applied basically the same as wallpaper but horizontally instead of vertically.
Prepare the wall by cleaning and let dry. Then apply the paper per manufacturers instructions.
Wipe off excess glue and let dry. Before hanging pre pasted wallpaper over the liner, it will need to be primed,
but even this extra step makes this the easier method for wallpapering textured walls.