It is better to clean only the dirty part of the wallpaper:
1. Mix corn flour and water.
2. Daub the mixture over the stain.
3. Leave it dry and scrub it until clean.
4. Repeat if necessary.
Grease stain will be gone if you dab baby powder to the stain with clean cloth.
Read manufacturer instructions before you clean the wallpaper. It is better if you try to find out how to clean the wallpaper from the wallpaper store before you buy and apply the wallpaper. There are many types of wallpaper in the market. Some of them shall not be washed.
If no instructions available, try to wash the small concealed part, such as the part covered with the bed:
1. Use soft detergent and warm water (not hot). Wash small part only, use a little bit of water.
2. Rinse, then slowly dry it with tissues or paper towels by letting them absorb the damp part.