The approach you take in making decals depends on the equipment available to you and the colour of the car you want to relivery. I'll try to show an overview of how I go about it. I have an HP Colour Laser printer, but you could use the same technique with an inkjet. I'm also going to assume that anyone reading this how-to has a working knowledge of an image editing application. I like to use Photoshop since that's what I use at work (I'm a video tape editor). I know it's not a realistic expectation but to include step-by-step what to do in Photoshop would take an even longer explanation than what I present here. Generally, there are three types of cars that would get reliveried: silver, multicoloured, or metallic coloured cars, cars of a solid colour, and white cars.
Silver cars are tough to relivery unless you have an Alps printer. These printers allow you to print white as a colour on clear decal film. They use a technique called dye sublimation which deposits material onto the film. There are Alps cartridges for the aforementioned white, as well as silver and gold metallics. If you don't have access to an Alps you can use off the shelf decals. The Monogram decals are a great starting point for metallic coloured cars. They have contingency sponsors as well as numbers and rondels. Pattos in Australia has hundreds of decal sets as well.
For a car that's a solid colour, I'll first photograph the car from different angles and make 1:1 printouts with the image partly ghosted out. I'll draw on that in pencil to get ideas.
The other thing you'll need to do is match the base colour of your printouts to the colour of your car. To do that, I printed out the Tamiya colour chart on decal paper to see how the colours reproduce.
It's a bummer to waste a sheet of paper on this, but it is a really important step. You need to know how colours will reproduce on your particular setup. It's best to do this with a sample of white decals with the white paper background. The blue papered decal paper won't help you calibrate your colours as everything will have a blue cast (from the backing paper - decals printed on blue backed decal film are fine). When you make printouts onto decal paper, tell your printer that you're printing transparencies. This setting in my printer's control panel seems to work best for me.
With your colour chart printout in hand, compare the colour on the chips to the colour of your car. Then use the colour picker in Photoshop to choose the colour closest to that of your car. The McLaren was a combo of Orange and Camel Yellow. You can use the transparent layer function in Photoshop to blend colours and print test shots on plain paper.
Once I've done my sketches and matched the base colour of the car I'll load the car images as a background in Photoshop and start pushing pixels.
Photoshop lets you turn layers on and off. I turn the background layer off when I'm ready to print and add my base colour as a background. I might also spend some time to rearrange all the decals in such a way as to more efficiently use each sheet of decal paper.
When you're working with paint or decals and ready to apply them, make sure your hands, work area and tools are really clean and free from oils.
Once I print the decals out, I spray the output with a layer of Krylon clear which is available at art supply stores. I'll let it dry for about fifteen minutes and apply another coat. I usually do 3 thin coats of the stuff applying each before the previous is totall dry. I like to get the acrylic soaking into the laser decal media and paper. The ink on the decal paper is really fragile, so anything I can do to bind it better is good. I've also had good results with Tamiya clear and Testors decal sealant.
I let my final coat of clear dry for an hour or so. When cutting the decals apart, cut with the blade of the scissor that you can see on the outside of each graphic being cut out. This will keep the pigment at the edge of the decal from flaking.
Once the decals are cut out, I dip them in water for 30 seconds (less for smaller decals), then set them on a paper towel to soak up excess water. While that's happening, I dab the area of the car about to receive a decal with Decal-Set. This helps the decal wrap around curved surfaces. By now the decal will have separated from the backing paper. I'll position the decal carefully with either tongs, a paintbrush or my finger, slide the backing paper out, and blot up any excess water. Once the decals dry I'll dab some Decal-Sol on to help them suck down into panel seams.
Once the car has dried I'll clean up any water stains and fingerprints and then brush on a three coats of Future acrylic floor polish with a sponge brush. This is done with the body off the chassis. I'll dab any accumulations of polish along the rocker panels. Future dries in about 15 minutes.
Here's another example of a car that has a solid colour paint scheme. You can see how I had to match the body colour with the printed background on white decal film.
Notice how the graphics are printed on the same colour as the area of the car they are going to be applied to.
The white lines are invisible when viewing the car on the track. It's interesting how photography exaggerates the smallest flaws. Decal Set helped Fernao's name settle down on that complex curve around the driver compartment. Decal Sol helped any bubbles lie flat.
White cars are the easiest to do custom liveries for since the white of the decal paper matches a pure white car quite well. I'll often print a test shot on plain paper and cut the graphics out to see how they fit on the actual model they are destined for.
I had to shrink the size of the Martini logos, as well as paint in the stripes for the final version of the Montini's decals.
Some alternate liveries for my McLaren above on that sheet as well. I try to fill each printout so I don't waste any decal paper as it's a little pricey at $4 a sheet for the Bare Metal Foil stuff. I'll lay everything out before I get started. It's best to cut all the decals out and have everything you're going to need at the ready before you get started.
Check out the Future-istic shine on the Montini:
The the pigment on the area around the hood pins on the Montini kept flaking off the decal paper. Rather than pull off the decal, I just laid another exactly over top the first. The colours get really saturated when you do this. The third layer of decal finally held fast.
The most important thing about this whole process is patience - especially when applying your home made decals. Since you're printing your own, if you mess one up when you're applying it, you can always cut another from your printout.
I hope this explanation of my approach helps readers create their own custom liveries.