Here are the steps for writing Lewis Structures for Covalent Molecules.

1. Add up the total # of valence electrons from all atoms in the compound.
2. Divide by 2 to determine the # pairs of electrons available to bond.
3. Determine the center atom, it’s always the element with the lowest electronegativity (closest to the left side of the Periodic Table). All the other atoms in the compound are end atoms. EXCEPTION, hydrogen is NEVER a center atom.
4. Draw the molecule, place a line, which symbolizes a bonded or shared pair of electrons between the center atom and each end atom.
5. Determine the # lone pairs of electrons left (only attached to one atom, not shared) by subtracting the # of lines you just dew (shared pairs of electrons) from the # pairs of electrons available to bond.
6. Starting with the end atoms, place the lone pairs (as 2 dots) around each end atom so that each one has a total of 4 pairs of electrons around it (lines + dots). This will satisfy the octet rule. If you have lone pairs left over after doing the end atoms, then place the remaining ones around the center atom.
7. If you run out of lone pairs before the center atom has 4 pairs, you will have to erase a lone pair or two from an end atom and make a double or triple bonded pair (two lines or three lines) between that end atom and the center atom.

Here is a video with some examples. They skip step 2 but the idea works out the same.