That one is not easy, but I did modified my current bumper. The bumper is originally made for Wira and I make it fit into my car. Modifying an existing bumper is not that hard, but starting from scratch, I have not master yet.
But modifying a bumper requires you to buy a lot and I mean really a lot of them and usually not cheap. To make a mould, you will need a design of what you want first, you can design either by using cardbox, thin plywood and finally putting a lot and a lot of putty for the finishing fine smooth surface. Then you will need a coating of paint, they will help us to prevent the fibre from sticking that hard to the design surface, but this is not really neccessary. Ok, then you apply the fibreglass as in the process i just showed, and apply many layers of them, around 2-3 layers of fibre clothes so that when it harden up, it will become a mould to that design, and a mould should be hard, really hard.
Ok let say everything is done, and you have the mould, you will need to sand the inner part of the mould to as smooth as possible, using sand papers and then finally with normal car polish material. With a mould ready, it will be an easy job, just apply the fibreglass into the smoothed out inner side of the mould and wait for it to dry, then you have it.
To custom fit a ready made kits is much easier alternatives, it will mostly require you to lengthen or shortened the existing size, and you need some brain twisting and storming of ideas to how to solve this matter, you know how the fibreglass is made, problem is how to use them. I cannot teach you here due to hard understanding, but remember to fully use your brain. So small stuffs, cardbox can be used to help design your stuffs.