How do you hook up a single pole thermostat to a base board heater

This is a straight 240VAC heater and thermostat? If so, you have 2 hot leads to the t'stat, black and retagged white. These go to the red, or input leads on the t'stat. On the output side, black to black and black to retagged white going to heater connections, ground to ground.

Is the thermostat mounted on the baseboard heater at one end? Since you said it is a single pole thermostat you must be suppling the heater with 120volts. Your incoming power should be a black and white wire. Connect the white wire to the wire from the other end of your heater element. Connect the black wire to one wire from the thermostat, connect the other wire from the thermostat to the wire coming from the heater element end closest to the end your working on.