The tail must start on a lone pair or a bond.
In this example, the arrow starts on a lone pair.
In the following example, the second arrow is drawn incorrect since it does not start on a lone pair or bond.
The head of a curved arrow shows either the formation of a bond, or the formation of a lone pair.
The head of a curved arrow can never show carbon forming more than 4 bonds or hydrogen forming more than 2.
Any curved arrow drawn should describe one of the 4 patterns discussed thus far.