i do a lot of my own decals. its not hard at all. after i get back form origins game convention i will try and write up a full article about decal work.
in the mean time, just get some decal paper from ebay. either inkjet or laser-jet depending on your printer.
decal paper will be clear or white. usually i use clear, but unless you own an ALPS printer, you wont be able to make white decals.
find an image you want and clean it up in photoshop or thegimp. and be sure to find the biggest cleanest image you can.save the image as a TIFF, tiff images are one of the best for printing.
open up a word document and import the image to it. then you can set the scale to get it down to size. usually under .5 " for a marine shoulder pad. now print this 1 image on regular paper and make sure its the right size. then weh you're happy. simply copy and paste the image all over the word document until you have enough.
leave some larger for tanks and such.
set your printer for the highest photo quality paper and max DPI. and follow the instructions on the decal paper.
after you print them, let it dry over night. then spray a coat of clear gloss over the decal. let dry and apply 2 more coats. evenly apply the coats!!!. let it dry over night. then cut out, soak in water and apply like a regualr decal.
here are some examples of my older work
http://st.cr.k12.ia.us/staff/jcobert/jeremys/mywebsite/images/CobraCommand/index.html">cobra
http://st.cr.k12.ia.us/staff/jcobert/jeremys/mywebsite/images/genestealercult/index.html">genestealer cult
http://st.cr.k12.ia.us/staff/jcobert/jeremys/mywebsite/images/praetorianguard/_0717.html">praetorian guard
i will be writing up a full article on applying decals, then creating decals soon.