Ernie, you've gotten lots of good suggestions; let me take a different tack on this (no pun intended, of course!):
As a college professor, I've written a couple of textbooks. I have found it helpful to work on the book *as* I am teaching the course. (Not necessarily writing it as you go along--I did that once and about went under!) But you will think of many, many things that need to be in the book as you are working with the students. If you don't write them down immediately (or keep a pocket recorder handy and record a memo for yourself) they will be gone when you sit down to write the book. You may even be able to work out some sort of release form that would let you take pictures of actual student mistakes for use in the book (probably need to consult a lawyer and your school on that one). At the very least see if you can take pictures as a way of reminding yourself of things you need to address, even if you have to reproduce the mistakes yourself for the actual publication.
I say all of the above even if you are not necessarily intending this book to be a "textbook" as such. The textbooks for the welding classes at the community college are more than $100, so they have a pretty limited audience. On the other hand, you could write a book for more general consumption that would still be very useful (perhaps even more practically useful) as a supplementary textbook, as well as for a lot of folks who are not taking classes (or already have taken classes). BTW, I don't know how publishers are in this sort of field, but textbook publishers in the humanities often want to see a *textbook* actually used a couple of times in a class, and preferably by more than just the writer, before they publish it. A more general-purpose text does not have to pass that test, so long as you can sell the publisher on it. As to the latter, publishers normally want to see that you have the credentials and experience to justify the book; I think you could refer them to any member of SEJW and/or RCM for a resounding recommendation!
Finally, I'll chime in regarding including a DVD or CD. More and more publishers see this as a way of distinguishing their books from their competitors. I think the words of caution about producing a full-length video (DVD) are worth noting. On the other hand, short video segments could sure make a difference in the usefulness of the text. I'm thinking for example about the short video demonstrating how to feed a TIG rod through your fingers that you posted to the drop box a year or so ago -- something like that would be *very* difficult to convey in text, but was clear as crystal with even a low-resolution video. Short video segments like this would, I think, be much more feasible and affordable to produce, and probably as useful and maybe even more useful than trying to put together 2 hours of continuous video.
Hope this helps!
Andy