Hi msmr,
Kep is approx 25km's away from Kampot (around 30mins). There's no need to book or arrange anything in Kampot. Just buy a boat ticket at the jetty in Kep when you arrive. At present, the boat cost is around $6-7 per person (assuming a full boat of six people). It takes 20-25 mins to get to Koh Tonsay. Accommodation there is cheap and basic but makes for a very pleasant experience. Food is good to as it's 'freshly caught'

Hope that helps. Enjoy!