In my experience, you have two basic choices. You can put a liner on the wall first (provided the texture is not TOO rough). This mutes the texture and provides a smooth (relatively) surface to glue paper to.
If you are using the services of a professional paperhanger, you will be paying for the liner and the hanger's time to put it on. On top of the price to hang the paper itself. And a paper with a delicate pattern may still show some of the irregularity.
If you want to save money and prepare the walls yourself, you can smooth them out by skimcoating the surfaces with all-purpose drywall joint compound.
This is do-able, if you have time and patience.
The first thing to do if you choose this option is to prep the surface. Remove any scaling paint, powder, dirt, etc. If you have any water stains, I would seal them by brushing on a stainkiller, preferably oil based.
If you wash down the walls, clean rinse after to remove any soap or cleanser residues.
I usually go over the surface with a study flat tool to knock off the worst of the rough points, if possible. This may help to reduce the number of skim coats of mud required.
Now you are ready to skim coat. Know that you will have to apply a minimum of two successive coats of all-purpose compound, or more if the texture is pretty rough.
Tools: a ten or twelve inch broadknife and a mud pan. Or, a plasterer's hawk and trowel.
There is a proper order to skimcoating, to ease your job as much as possible. The key thing is to be consistent. With each coat, you should be doing basically the same thing for the entire coat.
For example, you will want to go in the same direction as you travel across the surface, and not just slather the material on the surface every which way. Then when one coat is dry and you are ready for the next coat, you will put on the material going the other way.
I would recommend that you start the whole process at the top of the wall and in a corner and move across and down as you go. Remember, you will need at a minimum two coats. And try not to make the coats too thick. This is skim coat.
At this point, you may get by with a good sanding to smooth out tool marks. If not, do another coat, varying your stroke directions according to your judgement of what works best. The final step is sanding. Medium grit sanding sponges work well here.
Now that you finally have a smooth surface you can live with, be sure to put a couple of good coats of drywall primer/sealer (PVA) to render the porous surface fit for wallpaper.
If it's a new texture you're after, you may not need to prime before applying texture.
