What is the pre approval process for a home mortgage?

The title pretty much says it all. Just looking for general information on the pre approval steps for a home mortgage. I know it varies by state (i'm in ny) but just need a general overview: like how you start the process? how long it takes? what they ask/what you need to prove? Thanks
Update: Thanks for all the great answers! I do not have 2 years of work history (not even 1). I have 4 months with a very respectable employer. I have a student loan and no other monthy payments. I have about 27,000 for a down payment, if any of this info helps. I don't want to walk into a bank and be shot down-- i;m... show more Thanks for all the great answers! I do not have 2 years of work history (not even 1). I have 4 months with a very respectable employer. I have a student loan and no other monthy payments. I have about 27,000 for a down payment, if any of this info helps. I don't want to walk into a bank and be shot down-- i;m nervous to talk to them
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