Only use as much bondo as you have to. It is a good idea(like JuiceMoose said) to make sure that the scoop itself is securely attached FIRST via welding or screws or something. Bondo is nice because you can sand and shape it and redo it if you don't like how it came out or if you made a mistake. Will you be painting this too or are you having a body shop paint it for you?
use as little as possible,read teh damn insturctions and make sure it doesnt look like shit,im customing my honda out and im using soem bondo to shape it the way i plan,hope it turns out for ya man
go to a parts plus and get a putty knife/spreader. It looks like a big, wide plastic chisle. It will let you spread it on. Once you have the general shape, then you can sand it down to your liking.