How do Proctor, Francis and Giles plan to use Mary Warren's testimony to prove that "Heaven is not speaking to through the children?"
Mary Warren confesses to them that the girls are indeed lying. Perhaps pretending is a more accurate word, in fact, Mary Warren uses the word pretense with Proctor.
They get her to agree to testify, and she does indeed admit the same thing that she admitted to the guys, she uses the word "pretense" with Danforth. She turns on her friends and claims that they were pretending too. Proctor futher contends that Warren will "swear she lied to you." (meaning Danforth)
The manner in which they hope to do this is also through testimony. Giles and Proctor get together and determine that they have a common interest, the truth... oh and land. Thus, they convince Warren to remain faithful to that testimony. Proctor even envokes a bible story to remind her to hold fast to truth.
Giles connects this to others who on behalf of Putnam have been crying witchcraft just to get people's land. Giles gets peoples testimonies, has them signed and brings them to court as his own evidence.
The men try to prove that this is not Heaven speaking, but Thomas Putnam.