You can find the answer to this question near the end of Act Two in The Crucible. In this act, we are in the Proctor household. Mary Warren, their servant, came home from court that day and gave Elizabeth a poppet, or a doll, that she had sewn in court during the proceedings. Later, Elizabeth is arrested and charged with sending her spirit out upon Abigail Wiliams to stab Abby with a needle. It is a ridiculous charge, but they find the poppet, and in its belly there is a needle stuck into it. They conclude from this "evidence" that Elizabeth used the poppet as some sort of voodoo doll, and through witchcraft, stabbed Abby in the belly with the needle. Earlier that day, at dinner with the judges, Abby had fallen to the ground screaming, and pulled out a needle, "stuck two inches deep" in her belly. She claimed it had been Elizabeth's witch spirit that had done it. So, Elizabeth is arrested.
John sees through this weak evidence right away; he brings in Mary Warren and asks her if it was her doll, and if she had been sewing it in court that very day. Mary agrees, admits it was hers, tells them she put the needle in the belly "for safekeeping" so that it wouldn't get lost, and states, "Abby sat beside me when I made it." So obviously Abby saw Mary put the needle in the belly of the doll, and knowing that Mary worked for the Proctors, knew the doll would be in their house that evening. So, Abby takes advantage of the situation and stages her "stabbing," all in an attempt to get Elizabeth out of the way so that she can be with her precious John.
I hope that those thoughts helped clear things up a bit; good luck!