OK, I'll rise.....
You will have taken the basic DW module when you went through IOT, and most (but not all) branches also include additional modules as part of their professional training course - usually as part of a General Service Knowledge package.
There was a further DW element within the 3 week JOCC, but this was reduced to a point brief and a letter in about 1998.
However, the course referred to by the other contributors is of course ISS (Individual Staff Studies), which used to be a two module correspondence course but has been cut to one 4 month course (I think it's about 4 months) and a written exam, which must be hand-written rather than word-processed.
Not having reached the exalted ranks of Sqn Ldr, I can't confirm this from personal experience, but I understand that there are additional DW elements within ICSC and ASC.
SBG
P.S. Having read some of YWW's other posts, I now realise that he's an airframe tech, and clearly the above list applies to the commissioned among us. I'm not sure what is currently taught on IMLC etc.