Wooden slices or planks just cut them on the thick side. I wouldn't bother treating them as they will never last as long as stone or concrete, just replace individually as needed. Once layed down, make sure that there is no debris on top, these kind of walkways can be dangerously slippery when they get wet.

You won't be able to apply any home treatment that is effective. The likes of railway sleepers are pressure treated. I would lay them very slightly off level so that water runs off them - but not at such an angle that you can't walk on them - maybe 1cm (half an inch in real money) lower on one side.