Start kneeling on the ground with your weight on your hands and knees. Focus on keeping your abs tight and your body as straight as possible from knees to shoulders. Stop if you feel pain in your back or if your low back cannot stay straight. Use your arms to push your body away from the floor until they are straight. Then lower yourself down slowly and repeat.
To increase the difficulty, move your hands forward (towards your head) and/or closer together. Having your hands closer together puts more emphasis on your triceps and other arm muscles. Perform up to 15 reps and breathe out when you are pushing away from the ground. If this position is too difficult, you can perform them standing with your close to a wall with your hands on the wall. The further your feet are from the wall, the tougher the exercise becomes.
If this exercise is uncomfortable for your elbows or wrists, try taking out the up and down movement. In this variation, start the same way as before and start lowering your body, but instead of pushing back up, just hold the lower position. You can stop at any point of the movement and the closer you are towards the ground, the more difficult it becomes.
Safety Note: Push-ups can be hard on the wrists, so if they feel uncomfortable, you can use a different hand position. A common change is to perform push-ups on your knuckles instead of having your hand flat on the ground. You can also use padding, such as a towel or pillow to make them more comfortable. Use whatever option feels best to you.