Load a tube of caulk into a caulking gun. Pull the trigger several times until the plunger rests on the end of the caulk tube.
Cut the end of the caulk tube off with a utility knife to accommodate the size of the gap between the wood and the cement. Fill large gaps with backer rod to save caulk. Poke the inner foil seal through the open end of the tube to allow the caulk to dispense.
Fill the gap between the wood and the cement with backer rod if needed. Place the end of the caulk tube at one end of the gap, and pull on the caulk gun trigger to begin dispensing caulk into the gap.
Tool the gap, either with your finger or with a caulk finishing tool, to smooth the surface of the caulk and to force the caulk farther into the gap to create a tighter seal between the wood and the cement. Allow the caulk to dry and cure before exposing the caulk to water.