There are few borders you can use - below are detailed instructions from the Canby Publications website for the easiest route - you can grab a canby guide in kampot which will have the same instructions inside to carry with you -
"From Kampot, arrange a taxi to the border (Phnom Den) through your guesthouse. Private taxi costs about $25 and take 2-3 hours. If you like, the guesthouse can help to find other travelers to share the taxi with you and reduce the per person cost. The taxi will…

