Measure the cubic area of the room where the heater will be located. This number is the width times the length times the height of the room
Calculate the number of degrees you will need to raise the temperature for a comfortable environment. Get this number by subtracting the temperature of the unheated room from the temperature you find comfortable. For instance, if the temperature of the unheated room is 20 degrees Fahrenheit, and you would like the room to be 70 degrees, subtract 20 from 70, which is 50. This number is the number of degrees your heater will have to raise the temperature of the room.
Calculate the number of BTUs, or British thermal units, of heat your propane heater must provide. This is the total cubic feet times .133 times the desired increase in temperature. This is the amount of BTUs needed to heat your space.
Purchase a suitable heater for your needs that can provide the right amount of BTUs of heat.
Install your propane heater. Free-standing heaters should be set on concrete blocks or other non-combustible materials. Wall-mounted heaters should installed on non-combustible wall material.
Provide venting to the outdoors for vented heater types. Ventless heaters do not require outside venting. Consult the instruction manual from the manufacturer for safe operation procedures.