I've put a hood pad on my W108 since it never had one when I bought the car. It's an easy job but best to do this with a helper.

I didn't remove the hood at all, just opened it all the way like you normally would. I used 3M Yellow Super Trim Adhesive spray on the hood pad and underside of the hood (make sure you cover the engine and tops of the fenders to avoid any drips coming down from the adhesive on the underside of the hood). Once you've sprayed the adhesive on both, you and your helper grab the left and right side of the hood pad an align it as best as you can to the underside of the hood and press against it firmly all over to make sure the pad and hood have full contact evenly. Let it dry and you're done.

I've noticed engine noise is a little quieter and the heat from the engine and the sun combined on the hood will help prevent your paint from getting damaged in the long run.

Hope this helps!