It comes with little double-sided foam sticky pads and you stick it to the wall with those. God knows how I will ever get them off if I want to redecorate but they certainly make installation easy - only fiddly bit is working out how far apart to cut the slits for the brackets that fix the radiator to the wall.
you cut slots in it to fit over the radiator brackets. The slots start at the bottom and don't go all the way to the top.
If you have a radiator shelf, the top of the insulation will probably not show. It wil get quite dirty from the convection currents carrying dust past it. It might be possible to fix it to a clean wall using blobs of blu-tak, I haven't tried. It is only needed on cold external walls.