First of all, you should sit on the seat of your rowing machine and make sure that your sitting position helps you maintain your balance during exercise.
Now you should place your feet into the foot straps which are designed in a way to provide grip. Make sure you have adjusted your feet appropriately and firmly. Your feet along with heels should be on foot pedals. It will help you to use the strength of your whole foot not only toe which can cause you trouble in the form of injury.
After setting up your feet, you should hold the handle of the rowing machine in your both hands. Now use a little force to slide the seat of rowing machine forward ensuring that your both knees are up against the level of your chest. This posture is called the starting position of workout on a rowing machine.
Then you should use a little force for pushing your legs backwards and pull the handles with your hands up to your chest level until your legs become fully stretched or extended. This position is known as the power stroke.
After completing the power stroke, you should get back to the starting position which is called recovery step. Make sure that your knees are against your chest level in a perfect position.
You should repeat these steps from 4-6 times for completing a set and then take small breaks between multiple sets of this exercise. For getting good results you should consume at least 20 minutes in doing these exercises on your rowing machine.
If you want to make your exercise a bit hard then you can increase your speed or time which will help you to get greater level of cardiovascular fitness.