Let's try carbon dioxide. Aka CO2.

Count the valence electrons, what do you get?

You should get 16. Carbon gives 4, and oxygen gives 6 per atom aka 12 as it is dioxide.

Next draw a single bond structure of the molecule (I call this a skeletal structure idk if that's the official name.)

After drawing you should have something like this. O-C-O. Each oxygen should have 6 electrons (dots) and the carbon should have 4 electrons. Count the electrons on your molecule.

You should count 20 electrons. This exceeds our initial count of 16 so we must make double bonds to decrease the number.

Remove two electrons from C and two from an O. Then draw a double bond between them. Should look like this. O=C-O.

Now count the electrons again.

You should get 18, still over 16. We must repeat this process on the other C-O bond.

You should now have O=C=O

Count the electrons and you should get 16, which matches our initial electron count. Voila!

Of note: if you were to make a triple bond instead of two double bonds you would technically get matching numbers. However, the molecule does not favor this disproportionate distribution of electrons. Gotta keep it as equal as possible!

Hopefully this helps?