well i hope this helps...
I usually don't use too much putty, i got squadron putty and i've had it for quite a while, at least a year or two and its a small little tube. Well I'm gonna use my supra for the example. (you can find the progress thread called "my supra"). I usually make a styrene "skeleton" first. This usually consists ofr a way to shape the putty up to and around.
Part A is the outside of the front fender, you can place this as far out as you like using part B.
Part B is the inner fender piece, you glue this to already existing fender arch, with the wide side pointing skyward, so you have a sor of "edge" or "table" to add putty to.
Part C is the back pieces of the kit, this is if you want a back to the widebody parts rather than have the kit flow back into the bodywork.
I then fill in putty as needed and sand then add more putty and repeat the sand and putty process as needed. I hope this helps, if you have any more questions feel free to ask and I'll try to help.