The rules of the game
Atoms stay put. Only electrons (lone pairs and π bonds) move. Every curved arrow starts at an electron pair and ends at a new bond or atom. σ bonds don't move. Each arrow moves exactly one pair (two electrons) — for radicals, fishhook (one-electron) arrows are used instead.
The four allowed moves
(1) Lone pair → adjacent π bond. (2) π bond → adjacent atom (forming a new lone pair). (3) π bond → adjacent π bond (when conjugated). (4) Lone pair → adjacent empty orbital (carbocation stabilization).
Ranking contributors
The major contributor is the structure with: more bonds (versus charge-separated alternatives); full octets on every atom that can have one; smaller formal charges; negative charge on more electronegative atom, positive on less electronegative. Equivalent contributors (same arrangement, by symmetry) count equally.
Common examples
Carbonate ion has three equivalent resonance structures with the double bond rotating to each O — the real structure has bond order 1.33 on every C–O. The benzene ring's two Kekulé contributors are equivalent. Amide nitrogen's lone pair delocalizes into the C=O, which is why amides are flat and don't rotate around the C–N bond at room temperature.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.