I use the paper from micro mark most often, but I've used several other brands. It is best to use a good graphics/photo-editor to correct any problems. If available I just scan the d\kit decals, but if not available, or not the markings I want, I go to google images and find the images I need. Rescaling usually is necessary for these- pretty simple math.
Now print them out with an inkjet printer. I have used a cheap laser printer. But I find even inexpensive inkjets have better resolution and tonal shading than cheap lasers. There are also laser decal papers available- be sure to use the correct decal paper to match printer.
NOW-IMPORTANT! Carefully put the prints aside for three days. Then clear coat. I find Testors glosscoat or dullcoat works fine, but I find new clear acryics are good two. Put one a light coat first, followed by a heavier coat latter. Let dry a full day. You do need to cut out each decal- the entire sheet is one big decal. Then apply like any other decal.