This theory states that molecules for which two or more satisfactory Lewis structures can be drawn are an average, or hybrid, of these structures.
Here is the example of Benzene. There are 2 different ways of drawing the structure.
The hybridized structure is an average of the 2 Lewis structures. Therefore each bond is more like a 3/2 bond, not a double or single. This explains why all the bond lengths are the same and are not different sizes.
So if a structure can have a double bond on a different atom, it must be draw. Place a double headed arrow <=> between the 2. There may be 3 or 4 possible structures, this is a pain, but all must be drawn.
Here are more Lewis structures videos of molecules with resonance.