How can I draw the Lewis structure for ClO2-?

Here's how I would do it.

You can find the procedure .

For ##"ClO"_2^"-", "Cl"## is the less electronegative atom. So, ##"Cl"## is the central atom

The skeleton structure is ##"O-Cl-O".##

The trial structure is

You have 20 in your trial structure.

The you have available are: ##"1 Cl + 2 O + 1 e" = 1×7 + 2×6 + 1 = 20##.

Hence, the trial structure has the correct number of electrons.

The formal charge on each atom is:

##"Cl" = 7 - 4 - ½(4) = +1; "O" = 6 – 6 - ½(2) = "-1"##

Every atom has a formal charge.

We can reduce the number of formal charges by moving a lone pair of electrons from ##"O"## to form a ##"Cl=O"## double bond.

This gives us two new structures in which one ##"O"## atom has ##"FC = -1"## and the other atoms have ##"FC = 0"##:

The actual structure is nine of these. Rather, it is a resonance hybrid of them both