Well, you buy sticker sheets, you design your stickers in the software, print your designs onto the sheets via a printer, then you stick and slot it into the sticker maker, and it cuts around the designs (the cutting can be slowed for smooth cutting, and you can either add a white edge to your stickers or let it cut close).
It's easy enough to use the sticker making software (on PC). The actual machine is a little tricky, but once you've got the hang of it, it's easy enough.
The only things you'd have to look out for is good sticker paper and good printer ink. I didn't really have good quality gear, but I have a sticker I made about a year ago stuck to my fridge and it's stayed well (aside from fading, because of the sun).
(Most sticker makers also cut badges, those things you hook onto your door handle, birthday cards, present tags, vinyl, etc.)
To be honest though, I wouldn't recommend getting one unless you get a good response from what you want to sell or if you only plan on using it a few times. (< Like I did, and now it has been abandoned... xD )
It's expensive in small amounts, but, if you use it a bunch, it's worthwhile because you wouldn't be paying shipping fees and whatever.
I'm sure there's a lot of different ones, but mine costed probably a lot more than £300. That's without paper and ink, and maybe even the software.
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It was something like that.