To make the solution which is required to take off the already pasted wallpaper, you must add up two parts of hot water and one part of fabric softener. Mix them up in a spray bottle since you will have to spray the solution when necessary.
In order to saturate the wallpaper, you need to spray the solution on all the seams. Wait for a few minutes so that the liquid you have applied can settle down on the wallpaper. Keep repeating this step for some time. Eventually it will help you soak the wallpaper in the solution, which will help get rid of the paste behind the wallpaper.
The real work begins in this step. Grab a drywall knife and start scraping off the edge of the wallpaper right from the seam. Try to work in both directions as it is easier to scrape both the sides together. Try not to tear the paper throughout this time. Tearing it will make things worse for you. Therefore, just scrape till you fear that you might tear the wallpaper off.
Once you think that you might tear the paper, simply grab the spray bottle again and apply the solution to areas which are difficult to scrape off. Once you are done spraying and you have waited long enough for the solution to settle, scrape off again as much as you can.
Repeat the last two steps until the whole wallpaper is off. The trick is not to tear the wallpaper in order to remove the whole thing quickly.
Once the wallpaper is off, grab a sponge, soak it in water and wipe the wall with it. This will help you get rid of glue on the wall.