Do a seated lateral raise, and during the contracted portion, swing your arms around straight out in front of you. You then raise them straight up over your head before you reverse your actions. Bring your arms back down in front of you, swing around to the side, then lower back to starting position. The entire start to finish range of motion is considered one rep.
I use 10-pound dumbbells on these, so don't think you'll be able to go heavy. I typically do sets of 10 reps.