On each host profile is a button to send a message. Usually a phone number is also indicated (though some hosts choose not to communicate it).
Before contacting a host, be sure to check that all the conditions listed on their profile (length of stay, period of WWOOFing, etc.) are compatible with your project. 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 very based on the season). If you reach to an agreement, you may then clarify the details you want to know about your stay.