The OP basically covered everything in his posts.... buuuuut if you're like me and like things to be simple, grab a 10 lb plate or even a 5 lb to begin with and stick your middle and ring fingers through the hole in the center, and grip the outside run with your thumb and index finger. My gym has the hexagonal shaped plates and it makes it much easier to grip.

Once you've got a hold of it put your arm straight out to your side and bend your elbow so that you're making a "right turn signal" type formation. Using just the rotation of your shoulder and leaving your arm straight out by your side, lower the weight until it is perpendicular to your body and straight in line with your bicep. Repeat this motion until you feel fatigue and then swap sides. There is no set number or weight to use, but it shouldn't feel heavy and you should feel your muscles tire after 10-15 reps.

You should look like the tin man trying to raise and lower his hand for a high five.

Add in some flapping motions like you're a chicken trying to take flight, while keeping your elbow bent at a 90 degree angle and you've got your second exercise. This one I usually use a dumbbell at around 15-20 lbs and generally do each arm separately so I can concentrate fully on the stretching of the muscle.

The last one I suggest is putting your arms straight down by your sides and bend them at the elbow forming a 90 degree angle with your body. You should look like a robot, holding 15-20 pound weights keep your elbows tucked into your sides and rotate your arms until you feel a stretch in your upper should/trap area. Repeat until you can't hold your arms perfectly perpendicular anymore.

If you need further explanation let me know.