Step 1: Find valence e- for all atoms. Add them together.

S:6
O:6x3=18

Total=24+2=26*

*add 2 e- since it has 2- charge

Step2: Find octet e- for each atom and add them together.

S:8
O:8x3=24

Total=32

Step3: Gives you bonding e-. Subtract step 1 total from step 2

32-26=6e-

Step 4: Find number of bonds by diving the number in step 3 by 2(because each bond is made of 2 e-)

6e-/2= 3 bond pairs

Step 5: Find the number of nonbonding (lone pairs) e-. Subtract step 3 number from step 1.

26-6= 20e-=10 lone pair

Use information from step 4 and 5 to draw the lewis structure.

Lewis dot structure of SO₃^2-

Alternatively a dot method can be used to draw the lewis structure of BF₃.
Calculate the total valence electrons in BF₃ molecule.
S:6
O:6x3=18

Total=24+2=26*

Put sulfur in the center and three oxygen atoms on the sides.
Put a pair of electrons connecting the side atom with central atom.Put remaining electrons on the side atoms.Make sure each side atom get 8 electrons to get octet state.