Hi, the best way to volunteer is through an organization because most clinics do not offer this service officially but through volunteer companies. (You are not allowed to advertise medical services in Kenya).
I advice you to go with a local organization so that the benefits might be effective in a wider scale proportion instead of international companies who claim the owners of the projects while in real sense don't have the legalities to do so.
Don't be fooled by the sweet smooth talkers of international organizations who are just after your money instead of the well being of the societies they put you in, get a company that is connected with the volunteer project and serves the people.
I learned that in my experience last year.