Bend over, knees slightly bent, back nearly parallel to the ground. Hold weights at arms length (1 below), bars roughly parallel to your feet or slightly inward. Lift to the sides (2). Hold for 5 seconds before lowering to the start position. Repeat as required. Lift and lower weights smoothly, without either jerking or letting the weights drop. Check  safety and variations too.

Safety is important in this exercise. Bending forward without extra support puts strain on the lower back - remember all those "how NOT to lift a heavy weight" pictures? So this is not currently recommended early on, with heavy weights, or with damaged or weak lower back (you may be recommended similar exercises deliberately to train the back, but stick to professional advice). However, an important variation to reduce strain on the lower back is to rest one hand on a chair or similar while the other arm lifts, as shown in the related forward rowing exercise.

This exercise particularly works the muscles that your back uses to hold the bow straight, and to draw the string. In principle, it should be useful to get as close to the same movement and muscle use as when aiming. Try:
Adjusting the bow hand and arm position to mimic your shooting position. That typically means that a dumbbell would point downwards about 45 degrees.
Lift the weights as above, then move the weights in a small, slow circle at arms length before lowering. Mimics fine aim adjustment under load and also reaction to crosswinds.

Another useful variation: 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.