to do it properly you need the right equipment and correct filler rod. Pro is always gonna get the best finish but depends what the damage is.
I would diy repair a crack by maybe melting the rear together a little bit first with hot air blower attachment on my gas soldering iron and adding material at rear of crack. fibre glass the back of it for extra strength. dremel out crack a little and make sure there is a nice rough finish. then fill it with a flexible filller. flat it, prime it, paint it, lacquer it, polish it.
If it's not a major crack then you can use superglue as a repair and filler. Done it many times on various things.
First you must strengthen the crack from underneath. I always use chemical metal and key the underside for it to bond to. It sets fast and is rock hard or you can use fibreglass as suggested.
When it's set get yourself the superglue that comes in large bottles. They usually come with a special attachment for applying it in very fine areas.
Fill the crack with super glue and a little bit over then either let it dry naturally or use gentle heat from a hairdryer. Once dried sand it to a smooth finish. The bond should be nice and strong afterwards.
I did this on a completely buggered Mondeo bumper that I managed to rip off on a lampost when reversing cuz I wasn't paying enough attention.
It was cracked and split into several pieces. I bonded the whole thing with the method above and it was hardly noticible when I'd painted it.