I've done a some (ink jet most recently). I make decals only when I have to (either can't find the right decal or don't want to paint yellow or white stripes etc).
Whites are hard (either need white paper or put decal over white paint).
Don't apply too much sealant or you'll get a definite ridge on the edge of the decal.
Don't apply to little sealant or the pigment will run all over.
Don't cut too close to the image or the image will bleed.
I used Microsoft Powerpoint and MS Excel to creat the images.
I used Krylon Gloss Krystal Clear to seal the image.
Test print the images on regular paper first to make sure you're getting exactly what you want before you fire up the decal paper.
Make sure the printer options are set as recommended for the paper (I believe the instructions I had said to select the "transparency" option for paper type).
Some samples.
AIM 15mm US P-39 – shark mouth, eight ball, and "U.S. Army" not visible – on the underside of the wing.
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AIM 15mm P-39 (French) Roundels and unit emblem
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1/144 P-40 – roundels – a blue outer circle decal over a round white decal – you can see on the side of the closest P-40 that the blue decal bled a bit (inner right side).
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True North 1/144 Pe-2 – all of the windows are decals except for the canopy (ie the chin, side, and center top)
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These 1/144 scale fighters from the 325th FG have decals over a yellow-painted tail (True North P-47 and Battlefront P-40). I made the checker pattern with MS Excel
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AIM 15mm CR.42s – the white band on the fuselage is a homemade decal (I hate painting white)
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AIM 15mm Me-109Fs – white bands again
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AIM 15mm Stuka – yellow stripes/band
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AM 15mm Hurricane Roundels and stripes and Macchi C.202
the white stuff
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I did try to make some invasion stripes for 15mm Horsas but didn't have much luck (black stripe over white paint). The black decals bled, left tell-tale ridges, and the stress of applying them left little cracks in the pigment.
Hope this helps, Kevin