After reading up on the various methods for doing labels on buttons, here's what I found was the simplest path to getting vinyl decals on your buttons.
Situation: I ordered all concave Happ-style buttons (actually, these iL ones from Paradise Arcade: https://paradisearcadeshop.com/buttons-il-industrias-lorenzo/809-il-psl-l-translucent-with-black-concave-plunger.html,
including matching Player 1 and Player 2 ones). It was only after that I realized I wanted labeled buttons for admin panel purposes.
Most labels, such as those sold by GroovyGameGear and Ultimarc, are meant to go under a convex or flat button. Often above a diffuser, for LED lighting from below.
You can also order sheets of vinyl sticker labels from forum members; Pongo (http://forum.arcadecontrols.com/index.php?topic=111054.0
) and SlammedNiss (http://forum.arcadecontrols.com/index.php?topic=130477.0
) are two who have offered this service, and there are a few others. But these are sheets done with a stock design; I wanted labels that matched the frontend design I have done, down to the font. So I went looking.
X-Acto knife -- you probably have one, but they're cheap -- or even better, quality papercraft scissors
Cricut printable vinyl (10-sheet pack) -- $9.95 at Michael's, spotted at $7.95 online.
Black sharpie - again, you probably have one
Cutting mat or other surface you don't care about cutting into.
The printable vinyl sheets are meant to be used in a Cricut cutting machine, which run around $200 bucks. Basically, these are like printers that use cutting blade to outline vector graphics files. They're used for craft purposes, mostly to cut thin corkboard, magnet sheets, paper and cardstock for papercrafts, or vinyl stickers. But the sheets are also meant to go through a standard inkjet printer, and are thin enough to snip with standard scissors. They are adhesive-backed.
I printed this sheet on a standard Epson inkjet (I use a Workforce 845). Layout was done in Photoshop, with a grid set up at 21mm boxes (that's the size of the diameter of the plunger only on a concave Happ-style button), with a 3mm gutter. One sheet gets a LOT of button stickers, so I tried several varying layouts.
The ones that ended up working were the ones with thick black borders. I was going onto black buttons, so I didn't need to go right to the edge. The black printed on the vinyl simply bled onto the black of the button. If you were doing black onto white, you'd just invert; for colored buttons, I'd recommend doing the bleedat a color that matches the button. Cutting with an X-Acto knife, I didn't need to use the weeding tool I bought -- I am guessing this vinyl is thinner than the ones used by sign shops.
I did the first set with the X-Acto, but I find that for going around round corners, the scissors are actually cleaner. This is a fancy pair of paper scissors left over from my days in a letter print shop, though. YMMV with lower quality scissors.
Snip out the decal as close to the desired shape as possible, remove the backing, carefully position the decal, and press onto the button. Take a Sharpie (fine point probably better) and if you have tiny bits of white showing at the edges, touch them up.
Looks pretty good, and in tests the ink was colorfast with my fingers, even when sweaty. The vinyl is very thin, and doesn't feel raised much more than the P1/P2 buttons do.