I have purchased a tow bar for my V70 (1999), however it is not clear how to remove the rear bumper so that I can fit it, can you tell me how to do this please?

 

Look inside the trunk for nuts to remove.Most of the time they are hidden under foam circles that you will have to remove.
You will also have to drill the rivets on the end of the bumpers.
Some models also had 2 screw going from the bottom of the bumper to the top from under the car.

Then it should slide off.