19.1Start with the mapping spreadsheet — but make it richer

Yes, I would absolutely expect some sort of mapping document.

But instead of just:

Source	Salesforce
customer_id	External_Customer_ID__c
company_name	Name
phone	Phone

make it more like:

Source field	SF Object	SF Field	Type	Required?	Transformation	Null behavior	Key?	Business rule	Example
customer_id	Account	External_Customer_ID__c	String	Yes	trim	reject if null	External ID	uniquely identifies customer	C001
company_name	Account	Name	String	Yes	trim	reject	No	legal/display company name	Acme
phone	Account	Phone	String	No	normalize	omitted = preserve	No	primary business phone	3055551234
state	Account	BillingState	String	No	uppercase	null = clear?	No	billing address state	FL

The extra columns are where most integration defects live.

Salesforce's own data-loading guidance emphasizes mapping required fields and selecting the Salesforce ID or external ID used for upsert.