When to use it
Use it when writing necessary to remember it takes one c and two s letters, just as a shirt has one collar and two sleeves.
Example
N-E-C-E-S-S-A-R-Y: one c (collar) and two s letters (sleeves) confirms the correct spelling.
Common mistake
Writing neccessary (two c's) or necesary (one s). The rule pins down both errors at once: exactly one c and exactly two s letters.
