Buying All Cash With a HELOC

I'm planning to buy an investment property all cash with a HELOC on my primary to BRRRR it. Since I am not using a lender during the escrow process, what are the checklist of things to consider once a property is under contract? In other words, what specific things am I responsible for during escrow for due diligence and processing of paperwork that a lender would typically do? Further, how do I demonstrate proof of funds on a HELOC (ie, the equivalent of a pre-approval letter for a loan)?