To build your back, begin with 8-12 strict reps and go "heavy" enough to take the set to near failure.
The more horizontal your body, the harder it'll be. So you may need to begin with your feet on the ground and your body at 45-degrees, or you may be strong enough to elevate your feet on a bench with your body parallel to the ground. As you get stronger, progress by changing the angle.
When simply elevating your feet becomes too easy, add load with sandbags or a weighted vest. If you struggle to finish a set at the angle you've chosen, just adjust your foot position (mid-set) to an easier angle in order to finish the set.
Flex your abs to prevent excessively arching your lower back, which cheats you into a shorter range of motion. Squeeze and engage your glutes to keep your hips from sagging, and avoid using momentum or "kipping" the rep.
Use a complete range of motion from fully extended arms and protracted scapula at the bottom of the row, to fully retracted scapula and humerus parallel to the body at the top of the row. - Andrew Coates