This works for 28mm, 1/35, & larger. No printer required. ;-) I use acrylic-based paints throughout. The instructions are wordy, but are really quite simple. (Sorry, no pics.)
1) Use a pencil to lightly draw parallel lines on paper. They should be about 1/2 the width of the figure's eye socket apart, or 1mm or so also works. (If you are doing several lines, keep a large gap between them so you can easily cut them apart later.)
2) Paint white glue along & over the lines. Let dry. (The glue isn't used to make the decals stick, it's used to allow the decal to separate from the paper.)
From this point on, try to keep everything painted between the lines.
3) Paint a coating of clear varnish down the line. Let dry.
4) Paint white (glossy or flat) down the line. Let dry.
5) Paint a dark line down the center. (I use Burnt Umber.) Be sure to leave white visible at the edges. I find it easier to paint the line in with short strokes (and a very fine brush.) Let dry. ;-)
If you want to get fancy and have blue or brown eyes, use those colors in the above step, then follow up with the darker line right down the center. (You need a very fine brush and steady hand to attempt this.)
6) Almost done. ;-) Paint another coat of clear varnish over your line. And… (you guessed it.)
7) To use your new decals, use a pair of scissors to cut perpendicularly across your painted line (a sharp hobby knife does not work too well.) Aim to cut thin strips of equal height (so you have matching pairs.) A 'squint' is a more natural look than the deer-caught-in-headlights look.
8) Soak your strips in water until the decal lifts off. I use a sharp pointed hobby knife to lift and manuever the decal.
9) Place a tiny drop of white glue&water in the figure's eye socket, then set your 'eye' there. Shift them around so that the eyes look right. Let dry. If need be, touch up the figure's face with desired flesh tone (not usually a problem.)
Yes, this was a bit wordy, and might be too fiddly for some, but the results are more consistent, and you don't need to worry about trying to repaint an eye several times (and all the subsequent retouching that entails.)
Hope this helps some of you! ;-)