When you say "Should have done it", it means pretty much what you said "you were supposed to do it", and it generally implies that "you did not do it". But it is possible that you were "supposed to do it", (and) you did do it.

Should of done or should have done?

The phrase should have indicates a missed obligation or opportunity in the past. In informal speech, it is contracted to should've, not "should of." You should have (should've) called me!

When to use should have done?

We use should have + past participle to talk about things we regret. I got really wet walking home last night, I should have taken an umbrella. The speaker did not take an umbrella when she went out last night so she got wet. She regrets that she did not take her umbrella.