some heat mats come with thermostat built in, some don't. With ones that don't, the heat mat will have a power cord with a plug like you plug into any normal power point, and the thermostat should be the type that has a power point slot which you plug the heat mat into, then the thermostat plugs into the wall. You place thermostat probe on the warmest bit the snake can reach, and set the temperature to ~33c.
Make sure you run it for a while with a thermometer on there just to check temps are safe and stable.