What points should be kept in mind while writing resonance structures?

(N/A) Resonance structures are possible for molecules or ions that contain two or more atoms connected by multiple bonds (double or triple bonds) or have lone pairs of electrons on the atoms.
Each resonance structure must be a valid $Lewis$ structure. The total number of electrons in the molecule or ion remains the same for all resonance structures.
In resonance structures,the positions of the atomic nuclei remain fixed. Only the positions of double bonds,lone pairs of electrons,and formal charges ($+$ or $-$) change.
$A$ double-headed arrow $\leftrightarrow$ is placed between the resonance structures to indicate that they are contributing structures.