When playing the accordion, the straps should be behind your shoulders. Your left arm should be between the bellow-strap and the board - tighten that strap so that it won't cut off your arm circulation, but not so loose that your hand will slip all over the place.
You can play the accordion either standing up or sitting down. When playing standing up, adjust the shoulder straps so that the bottom of the accordion is on level with your hip. Don't slouch over too much, keep your back straight, so that the accordion won't move around too much.