How do you break down items in the game I got the perk but unsure what to do

You can do it manually via the workshop menu (items have to be on the ground) - but you don't have to - just dump everything in the workshop and it'll get broken down as needed.

For guns/armor, you can do it at the respective work station.

There are things you can't break down at the work benches- clothing, for example. You have to drop them out of your inventory onto the floor, open the build menu, highlight and scrap them.