It's not all that bad, but it definitely time consuming. Here is what I would do (caveat: I have not done this myself, so I'm speaking from theory, but I have done a couple of body mods to my car, so I'm pretty confident that this is correct):
Place the part on your hood in the spot where you want it to be.
mark a line around it on the hood with a sharpie. Using a grinder and sanding pad (80 grit minimum) grind from the line to about 4 inches out from the line to roughen up the hood surface (get all the paint off).
Using a bonding agent (several glues are available for 'glass to metal), stick the part to the hood. I would follow that up with either rivets or sheetmetal screws to be sure that it won't move around on you,
Sand the cowl a bit at the joint to the hood. Then, using some fiberglass reinforced body filler, skim the joint with a wide enough swath to make the transition smooth. Sand that down to feather the edges. Once you are happy with that, skim again with plastic filler, making sure that the edges are feathered to the hood surface. If you want, you can then skim with polyester filler to fill the sanding scratches. It may take several skims of each type of filler to get the desired results.
Like I said, it's not difficult, but is very time consuming. Expect to have it take you 3 or 4 full days of work. Then you can primer and paint.
Tim