I live in California and Social Security Benefits are not taxable. Where do I enter the adjustment for my Social Security Benefits on the State return form?

That is done automatically on 540 schedule CA.  The taxable amount of SS on 1040 Line 5b will be subtracted.

See CA 540 sch CA line 5  https://www.ftb.ca.gov/forms/2018/18_540ca.pdf

I made 2 screen shots for SS and CA.  

Federal 1040 showing taxable amount and California 540 (CA) showing it being subtracted out