For making decals, artistic ability is highly over-rated. I do a lot of my own, and I just download anything that might be considered "art work." There is, however, a bit of science and engineering to consider. Most important, your inkjet printer is designed to print on white paper. It can't "print" white. The ink colors are balanced so they come out correctly on a white background. Get the right decal paper for the job. For you, the first thing is to make sure you get INKJET decal paper. It's not that laser paper will not be as good, rather that it just won't work at all. Likewise, inkjet decal paper won't work on a laser printer. Next, you can get clear or white-backed paper. Although the printer will like the white paper better for color rendition, you will have to trim around the edges of every image, which is next to impossible for anything other than a rectangle with nice straight edges. I try to design my models so that I am putting dark letters on a light background, so that I can use clear-backed decals and not worry about trimming edges: For this trolley, I downloaded a font ("Gold Rush") and printed the lettering using Microsoft Word. I adjusted the margins so that I started printing as close to the top of the page as possible. (I printed test sheets on regular paper first to be sure of the fit.) I only printed a few lines, and then I took the decal sheet to a paper cutter and chopped it off below the decals, so I had a nice clean edge which I could put back into the printer for another run. Dark colors, light background, no problem. A more difficult problem is doing anything with light colors on a dark background. Here, I printed the sign on the same clear paper, but I painted a white rectangle on the wall of the building. When I applied the decal, all the colors came back: Again, the "art" was obtained from the Internet. For this one, I added the red border to mask the edge of the white rectangle and make a cleaner sign. To print the pictures, I import them into Microsoft Word, which lets me scale the size to fit my needs. Ideally, if you want a "factory paint job" look, you need to start with a smooth surface. I used a rattle-can spray paint for the trolley, or you could use a satin finish clear spray to smooth out a more rough surface. I used a coat of satin (or "semi-gloss") spray after applying the decal, to seal and protect it. For the weathered look of the Campbells Soup ad, I applied it directly to the rough surface of the building, and sealed it with Dull-Coat. I like the effect of applying decals to wood surfaces: Yet another downloaded image, this Moxie ad was added to a plank fence made from coffee stirrers.
Another idea for getting around the white lettering is a little cheat I did. You do have to work in a four color process, if you have a MAC it's no problem and a lot of PC's with the right software can work in CYMK. The customer wanted his RS11 painted in the Bloomer line, a real line in Gibson IL that uses a variation of the CB&Q Chinese Red. The logo and lettering were in a font that was not available so I used Illustrator to find a font that looked closed and printed white letters on a red square background on white decal paper. After some trial and error, I was able to print a decal that matched the Floquil red I had painted the locomotive with. One the decal was on and dull coated, you can't see it. The cab logo was a simple diamond and was also printed on white, but since it had a border, a straight edge made easy work of that.
There are several methods for making decals. Here are some threads that describe how others have made their own decals. Making Your Own Decals Additional search terms: decal decals water slide dry transfer scratch build detail diesel kitbash
