New investor here and I'm very much still leaning all this myself. That's said I believe I can give you a top level highlight without much issues.
First I'll say that my understanding is that off market and on market are not all that different. The main differences is with off market you have to handle getting the paperwork to closing and generally you're talking directly to the owner instead of an agent.
These are loosely reflective of the steps we take with our own off market leads.
Get to know your seller. Why are they selling? What are they expecting from you? How soon do they need to sell?
Get to know the property. What are the comps for the area? What repairs are needed? How old are the major parts of the house (HVAC, floor, roof, etc)?
Identify your strategy. Will you wholesale? BRRRR? buy and hold?
Run Your Numbers! Make sure you know every cost you can reasonably predict. Taxes, insurance, random repairs, marketing costs, holding costs, partner cuts, realtor fees, etc.
Financing. Figure out how you're going to pay for the property. Will it be a loan? All cash from you personally? All cash from someone else? Something more creative like Assignment or SubTo?
Get locked in. Get the necessary paperwork signed between the necessary parties to provide to the closing team. Not a lawyer so I'll leave it to them to explain that further.
Inspections, typically some of this happens when your getting to know the property but sometimes you can have an inspection built into your contract.
Closing, depending on your strategy this is either the end of your deal or just the beginning. My understanding is that closing happens a little differently from state to state but I believe it's safe to assume the process if pretty similar to closing an on market deal.
Anyways, I hope this helps give you a rough outline of concepts to consider with your off market leads.