If there's a larger fiber-looking washer, that goes on the commutator end of the armature first. If not just assemble the motor without any washers, and feel with your fingers how much end play there is at each end. Then disassemble the motor and place washers on each end roughly equal to the amount of play you felt... if you felt. When you're done, the motor should have just a little end play, roughly equal at both ends. It shouldn't feel like it's pressing against the washers at either side. If it does feel like there's play at one end but not the other, disassemble again and move a washer from the side with no play to the other and try again.