To start, you need to wash the wall with soap and water. Even if the wall does not appear to be dirty, this is a good idea. Any sort of dirt or dust can be bad for the sticker. The problem is that the adhesive will stick to this dirt or dust instead of the wall itself, and then the decal will not be able to stick to anything else. If the dirt is thick on the adhesive, you will not be able to get it off and the sticker will be ruined. Take warm water and soap and go over the wall completely.
After that, make sure that you dry the wall thoroughly. As much as dirt can harm the adhesive, water can do the same. This could even soak through and ruin the picture on the other side. You will want to start by drying the area off with a towel; then, you will want to put a fan in front of it. This way it will get as dry as possible before you try to attach anything to that area. Be patient and let it dry out completely before you set to work.
Finally, you need to wipe the wall down with a dry cloth before you put the decal on. You may think this is redundant, since you just got done washing everything, but it really is a necessary part of the process. The thing is, the warm water will sometimes loosen up hard dirt without removing it. This dirt could still be in place, but it will not be as firmly connected to the wall anymore thanks to the water. You can get it all off with a towel before you attempt to put up the sticker.
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