Seatpost clamp rack mount and a pair of p-clamps at the bottom. The clamp rack mount works best with larger frame sizes where the clamp is close to level with the rack. On small frames the rack front support arms will angle down sharply and pinch together very closely at the clamp and overall the rack will be a bit wobbly with heavier loads. There are rack mount clamps available that simply wrap around the post and don’t serve as the seatpost clamp. You can raise that type of clamp up on the seatpost to match the height of the rack and avoid the sharp bends to the support arms.
Unsolicited advice — put a small piece of frame saver tape around the seat stays before mounting the p-clamps at the bottom. When you eventually decide to remove the rack, your frame will still look pretty.
-Kedosto