Questions INM site doesn't answer about extending residence permit

[raquelita]

I went to the INM site but all it told me was that to EXTEND I should "bring documentation" without telling what. . . The INM person I talked to on the phone talked more quickly than I could absorb. Which of the following are currently necessary?

US passport and two copies of the front page.

In my case, the new, still current permit.

A letter stating what it is you are trying to accomplish.

Proof of sufficient income to support yourself. This would be a bank statement with your name on it for the past three months. . .

Proof of living in Mexico i.e. a phone bill or a CFE (electric) bill with your name on it or a letter from your landlord saying that you rent the property at a given address...

Money to pay the fee about 3,100 pesos (2012).

Will appreciate the info as I hope to avoid unnecessary work as well as a repeat trip.