There is a complete thread with pictures here: (there was a url link here which no longer exists)
How is it done? There are 3 basic methods in use today and 3 more that have been mostly abandoned except by 3rd tier companies.
The three modern methods include extrusion of gelatin emulsion onto support as shown in that thread, and by slide or curtain coating. It is all described in US Patent 2, 761, 791 granted to T. A. Russell of Kodak.