Converting primary residence to rental

[Vincent Z.]

I currently have a primary residence that I am planning to rent out. I am planning on buying a new 2nd property which I will owner-occupy once I close on it. 

I will not qualify for both mortgages at the same time due to DTI limits so I need the lender to count my current primary residence as a rental.

My question is what do I need to document for the lender to count my primary's rental income so it comes off my DTI calculation for the second mortgage?