I bought different brands of fret dots, both sticker-type and rub-on transfer type and in both cases, they wore off or fell off after a few weeks. $10 wasted. After that I bought a box of like 12 colors of Sharpie markers for like $12 and have used them as per below.
For ukes without fret markers on the soundboard for the 7th, 10, and 12th frets, I used a hole punch to make a template from an old piece of overhead projector film, which gave me a 3mm hole, and on the soundboard fret markers, carefully placed the template and then filled in with a white sharpie, and then let it dry.
if you have a steady hand, and trust yourself to make the dots free-hand, you do not need the template.
For side-dot markers, and for ukes with a light-colored neck I just use a black sharpie and make a small dot, and ukes with a dark-colored neck either a white or silver sharpie.
If they wear down, I just re-apply, as sometimes happens when using the Dunlop 65 fretboard oil. Not a biggie.
Also isopropyl alcohol (rubbing alcohol) is an easy solvent for nearly all sharpie inks, so if you mess up, it is easy to remove the bad marks with a Q-tip or cotton ball and then try again.