1. Lie. Say the DNA evidence found at the scene matches the suspect's, even if you don't know yet or there was no DNA evidence. This is mostly legal.
2. Maximize. Convince the suspect that the situation is hopeless and they might as well confess.
3. Minimize. Tell the suspect that what he or she did is understandable, and suggest reasons why they did it.
But sometimes cops do too good of a job. Sometimes they feed the suspect the facts, as you can see in this partial transcript of the interrogation of a murder suspect (published in the Washington Post) who turned out not to be the guilty party:
Shelton: “Did she tell you to tie her hands behind her back?”
Vasquez: “Ah, if she did, I did.”
Carrig: “Whatcha use?”
Vasquez: “The ropes?”
Carrig: “No, not the ropes. Whatcha use?”
Vasquez: “Only my belt.”
Carrig: “No, not your belt ... Remember ... Cutting the Venetian blind cords?”
Vasquez: “Ah, it’s the same as rope.”
A teenager who was confessed to smothering her baby has been exonerated because of how the police got her to confess.
You can read more about her case here.
And for more examples of false confessions, read this disquieting story in the New York Times or this piece from the ACLU.