Wiring baseboard heaters is as simple as adding a 20-amp circuit. Simply pull a 12-2 non-metallic sheathed wire from the electrical panel to the area of the wall you want to place the baseboard heater. Since baseboard heaters have a built-in junction box, you won't have to cut in a junction box to feed it.
In the case of a 240-volt baseboard heater, you'll need to connect the two insulated wires to a two-pole 20-amp breaker. Place a piece of black or red tape around the white wire before connecting it to the breaker. This will signify that it is a hot wire rather than a neutral wire. Of course, connect the bare copper ground wire to the ground buss.
In the case of a 120-volt baseboard heater, you'll connect the black wire to a 20-amp breaker. Next, connect the white wire to the neutral buss and the ground wire to the ground buss.