Just like your brokerage account, you associate each of your stock trades with a Realized portfolio.
You can name your portfolios any way you like. To maintain your privacy, Realized works without requiring any of your personal or financial account info.
After you obtain you trade history from your brokerage, save your trade history on your computer in a .csv (comma separated values) file format, using a spreadsheet template.
Realized will read any trade history file that you choose to upload. Again, you need not disclose any account numbers or passwords from your actual brokerage accounts.