Spells are incantations composed of one or more words that Witches use to alter reality and produce magical feats. In order to work, logically, a spell must be pronounced by a person with magical abilities.
In order to alter reality, a Witch, or a number of them, can chant spells. It is one of Witches' common powers. As mentioned by Wendy Beauchamp, spells already exist and Witches must use them in their true original form for it to work. Tweaking on the contrary often brings disastrous complications to the effect intended in the first place. However, some talented Witches were said to be able to craft spells of their own - Ingrid, in her previous lifetimes, was one of those gifted Witches.