Do not add water to the almonds while they are processing. Doing this will cause the almond butter to "break," and your almond butter will not come back together to form a smooth paste.
Optional ad-ins include salt, cinnamon, maple syrup, cocoa powder, etc.
This method will work with other types of nut butters, but if you want to make hazelnut butter, you'll need to remove the skins after the nuts come out of the oven. Just place the hazelnuts (about a ¼ cup at a time) into a clean kitchen towel and rub vigorously between your hands until the skins fall off.