When I start working with a business I follow a checklist of fundamental steps.
Redundant Contact Information
It's imperative that you maintain redundant communication channels with your assigned business contact, whether it's the owner, an executive, or a representative. Print out a copy of this contact list so that you have a physical backup immune from device or internet problems.
Accurate Locations of Interest
When something urgent comes up, you want to be moving to the right place while trying to establish communications. It's also more reliable to prepare an accurate address list at the start in order to avoid ever receiving a wrong address later on.
Complete Supplier List
People often underestimate how important it is to manage your business dependencies. You want to build an exact understanding of what the business uses. This process will sometimes yield crucial discoveries, such as finding out that the business domain name was going to expire soon, or that the electricity bills have been neglected for a while and in danger of service suspension.
This goes multiple ways. People in the business need to know that you're an authorized contractor. You need to communicate with stakeholders in order to align their expectations with what exactly you can and intend to accomplish.