You will need to tell them what processes and electrodes are acceptable, acceptance standards for weld quality, whether you are looking for full penetration or partial penetration on butt welds, what to do about arc strikes on the base metal, cleaning requirements after welding, maximum weld reinforcement height, etc. You need to find a good source of info for this, such as a book on sheet metal fabrication practices and an experienced sheet metal welder to talk to.

Here's a little info:

http://lincolnelectric.com/products/tech/migweldss.asp

You may also try contacting SMACNA to see what publications they have that will help, and check with the Sheet Metal Workers Local in your area to see if they have someone that can tell you what info the workers would like to have to do their job easier. Here's another good source of practical books on fabrication:

http://www.fmametalfab.org/

And another good source of advice:

http://www.weldreality.com/

Hope this helps,
Marty