Take 4X4 transport from Freetown to Tombo (a few hours depending on weather, road conditions). On Thursdays catch the public boat to Bonthe. From there catch a weekly boat to the Turtle Islands and bring camping gear.
All I know is that the waves get big in a country where there aren't many big waves. I never surfed it but heard of from a third party that it gets big and good.
Overview
Atmosphere
General
Whatever business you have in Sierra Leone, if you have a few extra days, check these islands and report back. There is a ton of potential but it takes time to get there.