It’s harder than, say using a ceramic tile. But if you’re doing it yourself, take all the tiles out the packets, and organise them by thickness, at least that way you know where you stand, and by using a solid bed of sticky can make it a lot easier for yourself in the process. If any are really thick, get a wallpaper scraper and a hammer, and you can actually split it to make it thinner. (Work your way across the tile before splitting it fully)
There’s lots of porcelain copies about at the moment, that are a really good replication of slate. Tile Giant (if there’s one near you) sell one called emulated slate, comes in 3 colours in 60x40cm and looks pretty good when done. Topps sell a range called Rhondine (spelling?) which is another good porcelain copy.
If you need any more advice, my emails in profile, I’ll be happy to help with any more questions etc.