Open Accounts (companies) or Contacts (people) and choose Import. Upload a CSV, or paste lines.

1. Choose the file, or paste one entry on each line.
2. Match your columns to the fields. The identifier and the name are required.
3. Confirm. The entries appear on the page and in the sidebar count.

Nothing is checked yet. Checks start only when an automation that uses the list is on. See CSV format for the columns, and Lists for how lists work.