The muscles of the forearm are critical for grip strength. It can be difficult to train these muscles without using weights because most people have little to no experience training these muscle groups without any weight. The simplest way to train the forearms using body weight is a technique called manual resistance. This technique requires only that you or a partner resist against a particular movement.
Using manual resistance for a wrist curl is simple and does not require a partner. Sit on a chair or bench and place one forearm on its corresponding leg with the palm up and make a fist. Place the heel of the other palm on the knuckles, and push down. Allow the bending wrist to be slightly stronger so that you are able to go through the entire range of motion. Repeat the process on the other arm.
Manual resistance for wrist extensions also does not require the use of a partner. Sit on a chair or bench with one forearm on the corresponding leg with the palm down and make a fist. Place the heel of the other palm on the knuckles and push down, allowing just enough give for the wrist to travel through its normal range of motion. Repeat the exercise on the other arm.
Manual resistance for the hammer curl is actually an isometric exercise, meaning that the muscles contract but do not change length, so the joint involved does not actually move. Stand up straight with one arm bent to 90 degrees with a fist out in front of you, palm facing inwards. Place the heel of the other hand on top of the thumb and index finger and press down. Do not let either arm "win" as this exercise is meant to be done without movement. Hold the contraction for 10-20 seconds and repeat on the other arm.