You must have an account on BitBucket.
Download SourceTree for Windows: https://downloads.atlassian.com/software/sourcetree/windows/SourceTreeSetup_188.8.131.52.exe
Step 2: install
Check Lauch SourceTree then click Finish.
Click I agree then Continue
Click Go to my Atlassian
Enter name of your organization, then click Generate License
Download file license. Now click Use an existing account in Step 4
Fill the login form or use login with Google.
Enter your username and pasword of your Bitbucket account.