Sit on a rowing machine, with knees slightly bent, and feet placed flat on the footrest. Take the v-shaped short grip attachment with overhand grip, and pull your elbows back. Exhale when you pull the weight in, inhale as you let it move away from you.
As always, do not lock out your elbows by fully extending your arms. Doing so leads to tendonitis. The key in performing this exercise is how you lean in with the weight and how you flex your back. If you are using lighter weight you should lean forward as you release the weight, getting a full stretch. If you are going heavy, rolling your back could result in injury. If you go really heavy, then don't move your back at all, keeping it straight throughout the exercise. The way you bring the weight in will be consistent. Pull your elbows as far back as you can and as far as it is comfortable. Pull your shoulders back and tyour shoulder blades together. You want to bring your fists just under your chest, with you thumbs into your lower chest. Bringing your chest out helps the contraction of your back. Squeezing your abs will help you protect your lower back by stiffening the core.
Rows could also be performed with a straight bar, taking a shoulder length grip. Doing so with an overhand grip will actually shift some of the burden onto the back of your shoulders, but will focus on the middle back. Taking an underhand grip moves some of the emphasis onto the trapz.