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