you'll want to make a plug and then a mould.
Plenty of tutorials on youtube for split part moulds e.g.
https://www.youtube.com/watch?v=b0iV9vEVaRM
You will want to make the mould from fibreglass with a gelcoat surface. Lots of prep, materials and time to do it right. No easy way to do it apart from a split mould IMO as there are a number of vent cavities to mould into the final part.
It sounds like you want to soft cast the bumper. Consider the size of the piece and the propensity of a soft cast to deform or get out of shape the larger the piece. A cupcake mould or cake mould is relatively small and uses the weight of the cake mix to evenly spread and hold the shape of the mould.
But a bumper is required to be hollowed out behind, so when the fibreglass is moulded into the plug, then the plug should be a solid shape to ensure the fibreglass is not deformed initially.
Say if you wanted to use plaster of paris or spray foam, then you'd need to consider the finish of the plug before application of the fibreglass. If you moulded into such a plug, you'd have to do a fair amount of finishing once the fibreglass shape has cured due to the poor surface of the plug.
There are pros and cons of doing everything, but at a minimum, make sure you have proper safety equipment such as gloves, googles/glasses and a quality dust/fume respirator as f/g and carbon dust is toxic and dangerous.
FYI - I am about to mould my RC bumper and bonnet soon so have been going through the options recently on the 'how to'. Happy to answer additional questions as I have used F/G for a number of years. Moulding is kinda of new to me, but not that big a deal either