Draw the Lewis structure of the molecule or ion
Count the number of electron domains around the central atom
Count each bond as one domain, regardless of single, double, or triple bond
Count each lone pair as one domain
Add the number of sigma bonds and lone pairs on the central atom
Match the total electron domains to hybridisation
2 electron domains = sp
3 electron domains = sp2
4 electron domains = sp3
5 electron domains = sp3d
6 electron domains = sp3d2
7 electron domains = sp3d3
For molecules with resonance or multiple central atoms, determine hybridisation for each relevant atom separately
For ions, include the charge when drawing the Lewis structure and counting electron domains
For atoms with expanded octets, use the total electron domain count to assign hybridisation
For atoms with no lone pairs, hybridisation is based only on the number of bonded atoms if all bonds are sigma bonds
For atoms in double or triple bonds, count only the sigma bond for hybridisation
Check the molecular geometry if needed to confirm the hybridisation assignment