First try went surprisingly well, but not perfectly, so of course I spent a day or two dialing everything in for future reference. The directions on the package leave out a few helpful tips.
Most importantly, the instructions stress over and over that a laser printer can melt the decal medium*, and so recommend settings that... did not work well in my particular printer.
I have a Color LaserJet MFP277dw, a popular choice for home printers right now (and I'm pretty happy with it). This printer has a low-temp fuser, and the recommended 'plain paper' setting for decals wasn't fusing the toner to the decal sheet reliably. The print tracked down the page (left side of the pic), and also tended to rub off after printing. The slower, higher-temperature 'transparency' setting works perfectly (right side of pic), and the print is durable to rubbing and scraping. Win.
Soaking and sliding
Secondly the instructions recommend soaking in hot water (good idea) for 20-30 seconds (in boiling hot water, maybe). I found a minute to be more like it, it depends on the actual temperature. Ideally, pull the decal out just before the edges start lifting from the backing.
The decal itself appears to be a vinyl of some sort. When it's hot, it's stretchy and pliable. When it cools back down, it becomes fairly stiff. In the usual 'building models' case, you probably want the label to conform to the target surface, so slide and apply while hot (being careful not to stretch the decal entirely out of shape).
For my use (meter scales) I want the decal to preserve its dimensions exactly. Letting the decal cool to room temperature before application works perfectly!
*The decal media is Papilio Laser Water-slide Decal Paper from texascraft.com. I've been using various specialty Papilio inkjet and laser media for more than ten years and have always been happy with them. These days it needs to be said: I've not been paid for any kind of endorsement, and have not been gifted any free or discounted product. I'm just a happy customer. There may be better stuff out there, but I've not bothered to look for it--- and that's coming from a perfectionist who's never satisfied with anything.