After breakfast, Mr. Collins goes over to Longbourn and asks to "speak to Elizabeth alone."
That's code for "I'm going to propose to your daughter now."
So, Mrs. Bennet clears her other daughters out of the room. Fast.
Mr. Collins gets down to business, proposing in a, well, businesslike manner.
He points out his extreme thoughtfulness in proposing to a Bennet girl because he's going to inherit their house one day.
He also points out that Elizabeth is poor and that he is generous.
Elizabeth tells him, Thanks, but no thanks.
Mr. Collins says, I get it! You really mean "ask me again later."
Elizabeth responds, No, I really mean "no."
Before she can sprint out of the room, Mr. Collins tells her that he understands feminine behavior, and he'll propose again soon.
Mr. Collins details his qualifications as a husband (there aren't many) and concludes by telling Elizabeth that he, again, doesn't believe her refusal is genuine. He thinks she's being an "elegant female."
In polite nineteenth-century British terms, Elizabeth tells him to get lost.
Mr. Collins calls her charming and says that she'll accept him after her parents pressure her.