1. Start with your dominant hand in a "C" handshape. (Think of making the letter "C" with your thumb and index finger, and your other fingers curled into your palm).
2. Move your hand forward, slightly downward, and then inward toward your chest. Think of a slightly curved path, like a gentle arc.
3. As you move your hand inward, you'll slightly rotate your wrist so that your palm faces your chest at the end of the sign.
Important note: The sign for "in" is often used in combination with other signs to indicate location or position. For example, "in the car" would be signed by signing "car" first, then signing "in" towards your chest.
If you're learning ASL, I encourage you to find a qualified teacher or online resources to learn the correct signs and grammar.