Have you checked whether there are Hubs about this? I wouldn't be surprised. . .
There are various levels of complication to doing this. If you go with 'textbook,' you plan your walk, mark it with stakes, dig it out, fill with probably 4-6" of gravel for drainage, and lay your flags--dry for simplicity, or with mortar for a solid, durable walkway.
At the other end of the spectrum, you can just lay out the stone where you think you want it and sprinkle loose soil around the edges to encourage the existing grass to build up the sod around the stones.
Obviously, the first way gives you a highly professional result, if done right; the second way is no-fuss, no-muss, but the result isn't comparable, looking (at best) rather "rustic."
I'm thinking that if you're buying slate you are imagining something much closer to the first result than the second. In that case, you should definitely do more research about what's involved in terms of time, skill, and money.