Summarize this article:

The next step in your progression is to do “self-assisted” push-ups. These are almost true one-handed push-ups, but with small cheats to let you build up more strength. First, lower yourself slowly to the ground with both hands. These push-ups are done level to the ground, rather than on an elevated surface.  Assume a position as if you were going to do normal, two-handed push-ups. Again, make sure to keep your feet slightly wider than your shoulders. Reach out — up and to the side — with your secondary arm, i.e. the arm that will not be supporting your body. The idea is to let this free arm “assist” in the push-up by taking on a small amount of weight, but to rely on it as little as possible. Over time, you will build up strength and come to rely on it even less. You can also place the assisting arm on a slightly elevated surface. As before, slowly lower your body until your chest is nearly touching the ground and your supporting arm is at an acute angle. However, be very careful about how low you go and how far your shoulder is extended because there is a risk of injury. Once you are in position, try to explode upward in a single and fluid motion.  You may have trouble at first with raising yourself. That is OK. Simply shift some body weight to the assisting arm. You can also try widening your foot stance. Again, keep your core muscles tight through the motion to create body tension and protect your spine. Keep your elbow in (no chicken wings), and pull your shoulder blades down and back Another move that can build up your strength and perfect your form is the “negative” push-up. This means focusing on the negative or lowering phase. At this point, you are nearly at the true one-armed push-up.  Use one arm for this maneuver. Keep your free arm behind your back. From the starting position, lower yourself to the ground. Move as slowly as you can and keep control over the movement. When you reach the bottom, place your free hand on the ground and push up. Continue with your set. Whether you try the self-assisted or negative one-armed push-up, make sure to switch it up and use your other arm. You can also alternate arms for each rep rather than doing a full set. It is important to use both arms to avoid creating muscle imbalances or differences in strength.

Summary:
Lower yourself to the ground on two hands. Extend your secondary arm outward. Lower and raise yourself. Try a “negative” one-armed push-up, alternately. Repeat and change sides.