If I were you, I would + (verb)

Here you are giving an example of what decision YOU would do given the circumstances. This can be in past tense or in a conditional present.

Here are some examples:

"If I were you, I would enjoy my vacation."
"If I were you, I would explain what happened."
"If I were you, I would continue working until it is done."
"If I were you, I would book my reservations now."
"If I were you, I would answer the question."

By adding 'have' after the word 'would' you are talking about something in the past tense.

Here are some examples:

"If I were you, I would have enjoyed my vacation."
"If I were you, I would have explained what happened."
"If I were you, I would have continued working until it was done."
"If I were you, I would have booked my reservations now."
"If I were you, I would have answered the question."