Well, this may end up sounding complicated, but it's really not. Yeah, it's a lot of steps, but every one is pretty simple.
First, get the right kind of paper, and think carefully about your decals. For an inkjet printer, you need inkjet paper, and for a laser printer you need....Aw, c'mon, why do I always see the same hands? Right! Laser paper. Well, duh...
Your printer can not print white. How do I know? Because you don't have an Alps printer. How do I know what kind of printer you don't have? Because if you had an Alps printer, you would probably already know this, and you wouldn't take your chances with some online train guy like me. So, you don't have an Alps printer, and you can't print white. With me so far?
So what if you can't print white? Well, that's a problem if you're planning to print a white C&O logo to put on a black hopper. You can't do it. So, we get back to the planning part. Think about printing dark items on light backgrounds. That is much simpler, and it's really a better place to start.
I use Microsoft Word to print my decals. For text, it's easy. Load the font, type the words. You can even pick the colors. For graphics, you can import the picture into Word (Insert-Picture-From File) and then size it the way you want it. Change the margins to get as high as you can on the page, and as far to the left as you can. That decal paper is expensive. Use a little, cut the top off cleanly, and you can use the same piece of paper again.
Now, Grasshopper, is the time for patience. You have decals printed on decal paper, and you just can't wait to put them on. Wait. Go to sleep, and dream of distant places, and whistles sounding through the hills. While your mind is dreaming, more likely of exams you forgot to study for and classes you went to in your underwear, your decals have dried thoroughly. Apply decal sealant, either a spray or Micro Scale's Liquid Decal Film. Go to work, or those classes which you hopefully really have studied for. If nothing else, put something on over your underwear. Return to your humble abode, and repeat the sealing process. Wait a few hours, or do the sleep-and-dream thing, or the classes thing. When everything is really dry, your decals are ready.
But is the surface of your model ready? It should be clean and very smooth, mostly. Use a gloss or satin coat for a smooth surface. If you want a really crummy, beat-up old looking decal, use a surface covered with Dul-Cote. (That's not sarcasm, by the way. I've put most of my decals on this way, intentionally.) Once the surface is ready, use a decal placement solution, like Microscale's MicroSet, to get the decal on smoothly. If the surface has naturally uneven texture, like a grating on the side of a locomotive, Microscale's MicroSol will help fit the decal down into the grooves. I use a small, cheap paintbrush to apply these and to nudge the decal in place.
Finally, seal the decal surface after it's all dry. You may find that the edges curl up, or the decal doesn't go down smoothly. Use a little more MicroSol to re-soften and smooth down the decal. Once it's totally flat and dry, give it some more gloss, satin or flat surface spray to seal it.
Here's a trolley I did with my own decals, on a satin-finish surface:
And this is a couple of posters I put on a wood fence, wanting it to look like the weather had gotten to it:
When I started this, I knew nothing. The first time I put one of my home-made decals in water, it dissolved completely, because I didn't know I had to seal it. It's a learning process, and each time you do it, you'll learn a bit more. Have fun, and let's see some pictures when you've got one you're proud of.