I've found that with an inkjet I simply can't get decals that look as good as the professional ones. If you want to test the waters, try the testors kit. You can order a CD for $10 or so (go to their website) and then you just need to grab some decal paper and the rattle can of sealant. I generally use 3 light coats of the sealant before I try using the decals.
For scaling, I always print on plain paper to make sure I have the right size, then print on the decal paper.
Often times I've found that I can't get fully vibrant colors like professional models will. I've found that if you double up the decal (copy and paste in the software, place it with a toothpick) it darken the colors up nicely. Good luck.
Groot