This tutorial is about making decals from bitmap photos. I do this often when building cars I've owned and many times I don't even have the car to reference for photographs.
1. Decide what you will need photos of and either take pictures with your camera or get on the internet and Google those you'll need. I'm going to use a 1969 Volkswagen Beetle I'm building as an example. I needed decals for window stickers, license plate, dash gauges, bumper sticker and rear lid VOLKSWAGEN nameplate. I sold this car over 40 years ago so I couldn't run to the garage with my camera. I think the bumper sticker I may be able to grab from a couple scanned slides I have. The rest came from internet images I found.
2. I use Photoshop to edit images but there are many other affordable graphic programs out there that do just as well. It is VERY helpful if the program has layers as this makes things much easier. I'm not making this a Photoshop tutorial but will give the steps I used to get things to size ... this should be easy to translate to your own program.
3. Once you have edited the image to erase backgrounds and isolate the part of the image to be used it's ready to be resized. I typically make two final files of all the decals ... one for clear decal paper and one for white decal paper. If any of your decals need white you have to use white decal paper. Hopefully these can be rectangular over all as you will have to trim the image right to the edges. I have one image, the VOLKSWAGEN nameplate for the rear engine cover that has to be on white paper and is impractical to trim. I'm going to try a trick promoted by Harry Pristovnik which entails making the background color the same as the body so only the name plate will show. I'll report later in this thread how successful I am at this!
4. To size the images to scale I first crop the image as close to the edges as possible. I then find the actual size of the decal I need in inches. This may be an actual measurement off the model with calipers or from the real world size. If you have the real world size, divide that by your scale. For instance, a US license plate is 12" wide and my model is 1/24 ... divide 12 by 24 which equals .5". So, the decal of my license must be .5" wide. When resizing images I don't "resample" the image .. I don't change the number of pixels ... just change the images size in inches ... the resolution (pixels per inch) will go up. These are Photoshop terms but yours may be the same. Just pay attention to the concept.
I get all the individual decals to this point.
5. Now you have to decide at what resolution you wish to print at. Usually 300 DPI (dots per inch) is adequate for most inkjet printers. I have a very high level inkjet that will print at over1500 dpi so I usually make the decals at 1000 dpi. This also helps low resolution images that break down at 300dpi. This is personal but I find it works fine. I create a new image that is 8.5" x 11" at 1000dpi ( or the resolution you want to use) to copy all the separate images to for printing.
6. Transferring all the images you have prepared in step 4 requires making the resolution match your print resolution image ... 1000dpi in this case. I do this in Photoshop by changing the Image Size with Resample turned on. I just change the resolution to 1000 so the image dimension remains unchanged. You will have to check how your program sizes images to leave the dimensional size alone and just change the pixels per inch to your print resolution. Now when you Copy (ctrl C) the individual image and paste (ctrl V) to your "all decals" image it will appear with the size correct in relation to all the other decals and the correct size for printing. I just arrange the decals along the top of my 8.5" sheet and print. Most of the time this is only an inch or so so I trim them off and use the decal sheet for the next ones I'll make.
Hopefully you can make sense of this text only tutorial so pay attention to the concept behind it all and you should have no problem sizing your decals and even using something different than Photoshop.
Here's a shot of the '69 Bug white decal sheet as an example .. with license plate, fuel gauge and speedometer. I always make a few extra just in case.