Sometimes the only thing keeping your LEGO model from perfection is a smooth, solid-colour face. Some model sets come with a set of decals that give that design just enough extra details to push them over the edge into realism. Making your own decals may seem like a daunting prospect, but it's actually not all that hard. Once you've got the design done, you're more than halfway there. You can find special decal paper on the web or at some stationery stores.
Sketch out the basic idea of your decal, or find an existing picture or danger decal (pictured) by searching on the internet.
Download and install an image editing program. The tools on a program like Microsoft Paint may not be enough to do everything you need to do with this image, and a program like Adobe Photoshop costs hundreds of dollars. You can find a free, more powerful image editor on the web, like GIMP (listed under Resources).
Start the image editor of your choice, and use the "Line" tool to make a 20-pixel-long line. Print this out onto a piece of regular paper.
Hold the 20-pixel line up to your LEGO model, and compare that to your drawing from earlier (if you did one). This will give you a sense of scale for how large your decal will come out. For more on scale, note that if a LEGO Minifig represents a person just under 6-feet tall, then each brick should be about 1½ to 2 feet tall.
Replicate or import the image of your decal into the image editor program.
Resize the image to be around the scale of the model. Your printer may vary slightly on how large the 20-pixel line comes out, so adjust your decal design accordingly. Make it larger or smaller than 20 pixels depending on how big you want the design to come out.
Print the design out onto a piece of regular paper again, (you may be able to recycle the 20-pixel page), and cut the image out.
Hold the sample decal up to your model. Look at how the design printed, as well as whether or not it looks good at that size and shape.
Return to the image editing software. If you're happy with the way the decal looked on paper, print it onto the decal stock (see Resources) and leave it for a few minutes to let the ink dry. If the image didn't look quite right, make some adjustments and retry printing it on regular paper. Repeat the adjustment and re-printing process until you have an image that looks right.
To print multiple decals, simply make multiple copies or images on the same page with a large enough gap between them to cut the decals out without damaging them. Decal paper is expensive, and you can't really print things out once you've cut into the paper, so you want to get as many decals out of one sheet as you can.
Don't touch the ink on the decal paper immediately after it comes out of the printer. You will smudge the ink and ruin your decal. It's a good idea to make multiple copies of each decal you want to put on the model in case one or two get damaged. Placing decals on your LEGO model may stick two or more bricks together. When you take these bricks apart, you will likely damage or destroy the decals. Take this into consideration when placing and designing decals.