Use a third-party folder-locking app such as Folder Lock, Wise Folder Hider, or Anvi Folder Locker
Right-click the folder, choose Properties, then go to the Security tab
Click Edit, remove unwanted users, and set permissions to Deny for access
Move the folder into a password-protected ZIP or 7-Zip archive
Use BitLocker to encrypt the drive that contains the folder
Use Windows file encryption by right-clicking the folder, selecting Properties, then Advanced, and enabling Encrypt contents to secure data
Hide the folder by right-clicking it, selecting Properties, and checking Hidden
Use a batch file or script to lock and unlock the folder with a password
Store the folder inside a protected user account with a strong Windows password
Back up important files before changing permissions or encryption settings