Wall heaters need a dedicated circuit so you will need to install a new circuit breaker at the main panel and fish cable to the location of your wall heater.
How to install a bathroom wall heater.
This video shows you how to install a wall heater with a wall thermostat.
If you buy a portable bathroom heater and if an electrical outlet is conveniently located nearby you can install it yourself.
You just place them away from water plug them into a wall socket and let them heat the room.
Attach wood pieces with screws.
Keep it away from places exposed to steam.
Moreover you should only install the heater on the ceiling if it is safe to do so.
Go through the attic to the bathroom fan and remove the electrical cable and clamp from the old exhaust fan.
After cutting the opening in the wall for your electric wall heater have an electrician install the new circuit and fish the cable to the wall opening between studs allowing slack to.
Cut pieces of a 2 foot x 4 foot board to fit between the joists blocking off the fan from the insulation.
Remove the 4 inch flexible air duct and then take off any fasteners securing the old fan to a ceiling joist.
The next step in your bathroom fan installation is to attach the fan enclosure to the joist with screws.
Completely remove the old exhaust fan.
If possible invest in one that is high enough to be difficult to touch.
Leave the electrical cable for use on the new heater.
Cut or push back the insulation leaving a 6 inch gap between the unit and insulation.
This unit has three heat settings you can use and a large fan which pushes the warm air farther into the room.
Do not try to save money by following the instructions on a how to.
Bathroom ceiling heaters or heaters that are installed a little high up on the wall are much safer than those close to the floor.
For all other types of heaters though and for all electrical work in the bathroom hire a licensed electrician.
Using a portable heater in your bathroom has many advantages one of which is not having to install them.
Resecure the electrical box in the wall replace the outlet and install the cover plate.