To make surfaces or things sit squarely flat horizontally, a carpenter's leveling tool is used for that. It is not common household tool so chances are you would not have one at home. How to make a home made temporary one is really easy. Take a glass or plastic jar with a cap. Draw a line about an inch high from the bottom around the jar. Put water in the jar up to the line you have drawn.
By setting the jar in the middle what you want to level, you will see what side needs to be elevated. The lower side will show water in the jar to be higher than the line. That should solve your problem on a surface that needs to be leveled. 
