Note that verbs that are used with まだ cannot be used in the past tense:
should be replaced by
The sentence means "I haven't done it yet", so since the state is ongoing, the progressive form has to be used.
まだ can also be used with the present tense:
Mada samui desu.
It's still cold.
Mada atatakaku naranai.
It hasn't become warm yet.