You'll find information on sizing for a Waterman here:
http://www.pensacs.com/New%20Pages/orderinfo.html
Installing is a matter of carefully removing the old sac and adhesive (gentle scraping; no chemicals!), cutting the new sac to the correct length, and gluing it in place with shellac.
-- Edited by Chthulhu on Sunday 28th of March 2010 03:42:46 PM
If you can hold off until I get my new repair lots in this coming week, I'll do a video how to restore a basic lever filler video for you.
Contact Giovanni at tryphon.it & get your general repair supplies. Martin at woodbin http://www.woodbin.ca/ has all of the sacs you'll ever need.
I have dealt with both of these guys for a decade plus & have never once been let down.
Generally speaking a 40's waterman will most often take a #16 size sac, a BCHR 56 and up take a #18. (fwiw, #16's are the most common size sac used in vintage pens)
if you can shoot a pic of your pen it'll give me a lot better idea what model you have. I have several watermans on the way in so chances are decent I may have the same pen for the how to video.
__________________
Tu ne cede malis sed contra audentior ito ~Yield not to misfortunes, but advance all the more boldly against them