lewisrules

Use only s and p valence electrons^*
Add or subtract electrons for negative or positive ions respectively
Do not create or destroy electrons
Use the following rules for the shared and unshared electrons about each atom

a. Rule of 8(2):
required for C, N, O and F
required for all terminal atoms
preferred for all others

b. When there are too many electrons for the rule of 8(2):
Possibility of 10 and 12 for n > 2
(Do not use with double or triple bonding.)

c. When there are too few electrons to fulfill the rule of 8(2):
Possibility of 6 (electron deficient) for B

d. When there are too few electrons to fulfill the rule of 8(2):
Possibility to use double bonding for C, O, N and S (rule b and d mutually exclusive.)

e. When there are too few electrons:
Possibility to use triple for C and N (and rarely O.) (rule c and d mutually exclusive.)

f. Use and designate Resonance Structures.

^* Using the d and f electrons is an advanced subject. It is best reserved for an advanced inorganic course or possibly a quantum mechanical course.