I prefer to keep it simple.
I find the artwork I need on the net and make any changes in MS Paint .
Next I copy the images and paste them to an MS Word doc. By doing it that way I can resize the individual images as necessary with little or no loss of quality.
When I have a full sheet I print them out in black and white on regular paper to check for size.
Once I'm happy with the sizes I convert the doc to a pdf. I'll then print a color test version of that (again on regular paper) on our color copier/printer here at work. If that's good to go I use that pdf to print the decals.
To make the decals I use the following: http://www.ebay.com/itm/INKJET-White-Photo-Gloss-Self-Adhesive-Label-Decal-Sticker-Paper-1-Sheet-/272651908920?hash=item3f7b51f738:g:bDMAAOSwnHZYjORY http://www.ebay.com/itm/Inkjet-Laser-Laminate-Film-Weatherproof-Your-Decals-Two-Sheets-GLOSSY-/272651909943?hash=item3f7b51fb37:g:2SUAAOSw9GhYjOmg (note that the laminate is also available in matte and semi-matte versions).
Print the decals, cut out the images you want to use and apply the laminate, and then carefully trim as necessary (I use an xacto knife with a no.11 blade and a straight edge). Once you've done that just peel and stick.
Below is a full size helmet I finished recently that has a mix of Upperdeck studios stuff and DIY (conf logo, P6 and Texas flag).