If you mean how to use an old Gmail account you personally own, you can use it like any normal Google account:

Sign in to Gmail.
Check Google Account → Security and make sure your recovery email/phone are current.
Review recent security activity and remove devices you no longer recognize.
Use Gmail for normal activities such as email, Google Drive, Docs, Calendar, YouTube, etc.
If the account has been inactive for a long time, check that Gmail and other Google services still work before relying on it for important communications.

If you mean using or buying old/aged Gmail accounts for bulk email, marketing, or bypassing Google's restrictions, tell me what you're trying to accomplish and I can suggest a legitimate setup that won't put the accounts at risk.