Count the number of valence electrons by adding the atoms' group numbers together. For example, for carbon dioxide, CO2, you would have 6 + 2(8) = 22 electrons.
Connect the atoms with single bonds, or lines. For the above molecule you would have three atoms (C, O, O) connected with two bonds.
Add the remaining electrons to your Lewis structure by adding multiple bonds and lone pairs. In the above example you would need two double bonds and four electron pairs.
Count the number of atoms bonded to the central atom then add the number of any lone pairs on the central atom. In the above example, two atoms are bonded to the central atom and no atoms on the central atom so 2 + 0 = 2.