There is a button to send a visit request on each host profile.
Before contacting a host, be sure to check that all the conditions listed on their profile (length of stay, availability, etc.) are compatible with your potential visit. Introduce yourself briefly and explain to the host why you picked them in particular. Ask whether they will be available during the time you want to come, and which activities you will be able to contribute to (it can vary based on the season). If you reach an agreement, you may then clarify the details you want to know about your stay.