On your first placement match you get matched with players of similar skill from qp stats then let's say you performed really well on your first placement match. The game places you in a slightly higher match next, no matter if you win or lose. Let's say you performed bad on your 5th match, the game places you in a slightly lower ranked match on your 6th. The rank of your final match determines your rank.
If you played before, it just puts you in a game of your rank from previous season and the rest is the same.