I've bought numerous characters for my V3 and Steph3 figures in the past but would like to have a go at creating my own.  Do you know where there's a tutorial that explains what all the body morphs mean or do? Looking at all the muscle groups  is a little baffling unless you happen to be an anatomy student, I'd like to know how to use them to build a figure to my own design.

Probably the best bet is to buy or check out at the library some body building
books.  They would explain how to build up each muscle group, but that
way you could see what that muscle group looks like and the proper name
for it.  It would also act as a guide so that you could shape your character
to look like one of the models in the book.  Then just start turning the dials
until your character's muscle looks like the one in the book.

I just started by spinning the dials to see what came out.

 If I wasn't sure what a particular muscle group did,  I just set the dial at 2 and  looked at the exaggerated version. Then I picked up some of Peter Levius's sets here (Illona also has a really nice set), imported the default poses as backgrounds, and  tried to reproduce the body.

get yourself a copy of "Gray's Anatomy" by Henry Gray F.R.S. with drawings by H.V.Carter.

The anatomical bible!

You'll probably the DAZ3D Morph sets for V3, but then there really are'nt too many really strong looking muscle morphs.  Look for the Muscular 1 - 4 morphs, starting from the Body.  They may be name PBM_Muscular 1 - 4.  I'm not at my computer now so I ca'nt look it up though.  After you've set the Body, go ahead an tone each body part seperately with the morphs after the same name, in each part. 

http://www.daz3d.com/shop.php?op=itemdetails&item=2252

http://www.daz3d.com/shop.php?op=itemdetails&item=1561

The pop-up pictures on David's and Michael 3's body morph pages at DAZ shows various muscles which pertain to a lot of the morphs.