Bend over, knees slightly bent, back nearly parallel to the ground, one hand supporting you and the other holding a weight at arms length (1 below). Lift to the side (2). Hold for 5 seconds before lowering to the start position. Repeat as required with the same arm, then change sides to train the other arm. Lift and lower weights smoothly, without either jerking or letting the weights drop. Check  safety and variations too.

Safety. Bending forward without extra support puts strain on the lower back, so a support is recommended here for isolated shoulder training.

This exercise uses the muscles that hold the bow straight and draw the string. The action is closest to that of the string arm.
To work the trapezius and deltoid more, move the weights forward (level with your eyes); moving the weights lower - nearer the waist - will work rhomboids more. High elbow shooters will tend to use upper trapezius and deltoid more, while low elbow shooters will use more rhomboid. But note that all good archers use both.
The same exercise is often used with two-handed lifting and without support to add lower back conditioning. I cannot recommend this without proper supervision because of the risk of lower back injury (above). It does, however, represent a useful variation if used with due care.