If you have a large area of wallpaper to remove, I would suggest the use of a wallpaper stripper. However, a wallpaper stripper is just a giant kettle and uses a lot of electricity to run so you should try and minimise the time it is switched on.
First, peel the top layer of the wallpaper with your painter’s knife/scraper (it should come off easily otherwise go to the next step) then use the wallpaper stripper to help you remove the bottom layer of the wallpaper, the trickiest part to remove. Scrape with one hand while steaming up another section of wall with the steamer on the other hand. Only scrape the wallpaper for as long as it takes the steamer to wet the paper. This will not give you time to remove all the bits of wallpaper. Then, when all the paper is wet, switch off the steamer and finish removing the smaller pieces of paper that are still stuck on the wall before they start drying out again.
This will not only help you save you electricity but will also reduce the time the walls are subjected to steam, reducing any potential damage to the plaster.
If you do not have a wallpaper stripper, just use a bucket of warm water and a sponge or kitchen cloth then dab the wall until the paper is wet, being careful not to dribble water over switches and sockets.