If you have proper alchemy materials you can make poisons. Just be sure to read the effects each ingredient does. If it heals you then you know that the potion will end up healing you. If it damages you in some fashion then you know it will damage another thing or person. Usually the potion will be poisoned if the icon appears in a green vial instead of the healing potions which are in purple vials. Possibly pink(depends on how you look at colors). In order to equip the poison you just go over to it and press what ever button you have for equip(I have no idea what system you are on). The choice will come up to put the poison on the weapon you have equipped. So theres a full basics course of poisons.
Murphy's Law: It is impossible to make anything foolproof because fools are so ingenious.