It's not really a big deal - you can wash it like any other part of the car. I'll wash the top surface of the transparent cover first with automotive shampoo through a pressure washer, then open it fully, pressure wash the undersurface with shampoo, and then spot-free rinse from the highest point of the open cover, downwards. I'll then rinse the entire engine bay with low-pressure spot-free rinse, close the engine cover, and rinse with spot-free (I use a CR Spotless rig, but you don't have to, if you dry the cover off immediately after, to avoid hard water spots).

You can get shampoo in the engine bay - just rinse the entire bay well, and then rinse with spot-free or blow it out with a leaf blower or similar.

Finally did my first detail....