You could use 6mm, you wouldn't see any deflection from that thickness. I've made a few rimless nanos that are 36 long and 8-10" tall with 6mm. Use 3/8" if you want overkill, but I personally wouldn't spend the extra $, it's a big difference in price.
It's simply a matter of siliconing the glass together. Use masking tape to hold everything together (not clamps) and have a friend help if possible, other wise you'll have to figure out a way to support the first pane as you attached the other ones. You can also use masking tape to define a straight seam and avoid making a mess of it, just remember to remove the tape before the silicone drys. Don't get stressed out over things like fingerprints, excess silicone is easy to clean up when dry, and don't clean up the excess silicone on the outside seams, let it dry and cut it of with a blade.
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