the wood type needs to be on the safe wood list first of all, and then needs to be scrubbed, boiled, and baked. i have yet to do my own wood prep, so i don't know the exact time needed in the oven, but hopefully others will chime in with that soon. i know that thicker pieces would need more time in the oven than say, twigs.
the biggest thing to remember is that whatever tree you get your wood from, that tree must NEVER have had any pesticides or other chemicals applied to it. that stuff stays in the tree forever. organic orchards would be a good source for wood.