When to start
Start when you sign the lease. Your organizational NPI, tax ID and state license have to exist before most payer enrollment can begin, and each payer then runs on its own clock.
The order is entity and license first, then NPI, then payer applications as soon as each has what it needs. EHR and clearinghouse setup run alongside. One rule is fixed: no claim goes to a payer until that payer confirms enrollment and an effective date.