Open accounts with vendors that extend net-30 terms AND report to a business bureau (e.g. Uline, Quill, Grainger).
Reported on-time payments are the raw material of a business credit file. Tradelines that don't report do nothing.
Open accounts in the entity's name/EIN, buy things the business actually needs, and pay on time. Confirm each vendor reports.
Source: KB.reportingVendors (confirm current reporting with each vendor)
The cost of things the business actually needs
A credit profile is time plus consistency. There is no honest shortcut — this is the step scammers sell around.
This is general business-credit education, not financial, legal, or tax advice. Confirm specifics with a licensed professional and the official sources directly.