When Teferi's Tutelage enters the battlefield, draw a card, then discard a card.
Whenever you draw a card, target opponent mills two cards. (They put the top two cards of their library into their graveyard.)
You draw a card and discard a card all while the first ability is resolving. Nothing can happen between the two, and no player may choose to take actions. This means that the second ability is put onto the stack after you’ve finished resolving the first ability.