"If I were you I would do it." This refers to the future. The person still has the choice to do or not do "it". It could be interpreted as "You should do it." "If I were you I would have done it." This refers to the past. "It" has or has not been done, but in either case, it's over with. Depending on the context, it could be interpreted as "You should have done it."

1st - future 2nd -past

'I would do' I would do XYZ in your position. It can be used in a multitude of ways. I mainly use it to imply a future tense and/or an imagined scenario. It can also be used as advice for the present. Me: I want to go to France. You: If I were you I would go. (if I was in your situation I would go) Me: I should really do the weekly shopping now. You: If I were you I would do it. (yes, you really should get up and do it) --- 'I would have done' I would have done XYZ in your position. This suggests that the XYZ has already been done. eg. XYZ is a test. Me: I didn't pass the test. You: I would have done more studying if I were you. In this scenario you're saying 'if this was me, this is what I would have done.'

in the first case his tip can still become reality,he can still do it. That's not the case for the second sentence,where there is no possibility anymore to do it.