My advice ..

Use the Long lengths of plaster Gyproc coving only, makes a much better job than the poly type!

There are 3 sizes .. Use the one that suits the wall height
You should remove existing paper on the area it covers.
Take the template included and mark a line around the wall parallel from the top edge. This is a height line you can work to.
Tap small nails (I use galv slate nails) into the plaster on the line to help support when you stick it on.
Mix the adhesive up .. I butter the edges along with a building trowel.
Use your finger to go along the joints ; )
Cut the mitres (escpecially the outside corner ones) carefully using the included template for that size of coving.
Pretty easy .. and using the long lengths will make it fast to do.
Adhesive is personal choice .. I use Coving adhesive - big bags ... dont mix too much at once, cos it goes off quickly! I mix to a thickness that will just about stay on a trowel .. not run off!

Have fun!