counter sinking is probably the easiest - drill one hole to fit the shaft of the screw, and then switch to a larger bit to make room for the head.
I've used nylon screws, but you either have to use some sort of adhesive, drill the whole slightly too small (in the wood) and friction-fit, or tap the wood (I already had some, and this worked wonderfully). Most use stainless wood screws, which would be easier and work just as well
Stay away from anything like concrete/cement/etc., it will screw with your pH, GH, KH, dissolved solids, etc.
If you don't want to use a screw, use some type of epoxy, it's probably the best all-around adhesive. Epoxy creates a really strong bond, is great for porous items (like wood - superglue isn't great for porous items), can cure really quickly, it's inert/non-toxic, etc.
easiest place to get slate is the floor section - just get a loose tile at Home Depot/Lowes.
I put my slate completely on the bottom, and then cover it with substrate, so it's not really any different then the rest of the tank, and I don't have any issues with plants growing over it. This also means it's much less likely to shift, and will be more stable long-term, but it also means you have to plan it out from the beginning.
Last edited by lochaber; 09-05-2013 at 09:48 PM.
Reason: added more info to cover posts I missed