When it comes to papering behind a radiator, it is not normally as difficult as it may at first seem, the key to achieving professional looking results it to take your time, be patient and if you need a hand ask someone to assist you.
The best way to decorate behind a radiator is to remove the radiator from the wall. If this is not possible then make sure the radiator is switched off and cool.
Measure and make a note of the position of the radiator wall brackets, measure from the edge of the radiator and up from the skirting.
Hang the wallpaper as normal but only brush down the part above the radiator, let the other gently rest against the front of the radiator. Measure where the position of the radiator wall bracket will come on the paper and mark with a pencil.
Cut a vertical slit from the bottom of the paper to the top of the bracket position, then lift the paper and drop it down the back of the radiator either side of the bracket. Use a radiator roller to smooth down the paper then trim at the bottom as normal.
If you don't have a radiator roller, use a long stick with some clean cloths attached to the end, to smooth the wallpaper behind a radiator.