The manual won't tell you how to clean it, however many people around here have had experience cleaning old boards.
Found this image
on Flickr of your board taken apart:

To clean it, it would just be a matter of removing the screws and opening it up like that and carefully removing all the key caps. The case pieces (top and right in the pic) could be soaked/scrubbed with a solution of dishwasher powder. The key caps should be soaked either in the same solution (with warm water) or even better a bath of warm water with some denture cleaning tabs thrown in. The plate with switches (bottom in the pic) should be carefully dusted or airblasted.
This is a pretty non-invasive process and it should work perfectly once the case and key caps are put back together again.