Make sure you press your tray evenly and deeply onto all of your teeth - including the back molars. This means pushing with your fingers not just onto the front and middle, but also the back of the tray when doing your impressions.
Make sure you do not move the tray sideways when pushing your teeth into the putty, or when removing the tray, or you will get 'drag marks' or 'double imprints'. Also make sure your putty goes all the way to the back of the tray or your teeth won't have anything to sink into.