« Tirer les plans sur la comète ».

Literal translation:
To draw up plans on the comet.

Meaning:
To count one’s chickens before they’ve hatched.

Tu veux être médecin, génial ! Mais je crois que tu es en train de tirer des plans sur la comète. D’abord, tu dois obtenir ton diplôme.
You want to be a doctor, great! But I believe you are counting your chickens before they’ve hatched. First, you have to get your degree.