Click on the announcement you are interested in. Check the date of the publication (some opportunities might be already expired). If the announcement is still valid, look for the ‘contact details‘ section. There you can find ways to get in touch with the hosts (website, e-mail, phone numbers, application form). Send the host/organization a message and wait for a reply.
Some of these opportunities are free(you receive free food and accommodation in exchange for voluntary work – wwoof style). Others require a daily/monthly contribution fee for the food. Remember that many areas of Central Africa are still developing and volunteering can’t always be a ‘free’ thing.