Steps for you
Make sure you are using a modern document library.
Right-click above the folder you want to share, and click Share
From the pop-up that appears, choose Specific People, then click Apply. This is the same screen where the user can allow or not allow editing of the folder.
Type the external user’s email and hit Send. You can also add a personal message if you wish.
OK, your part is done! Go and pad yourself on the back!
Steps for the recipient
The external user gets an email that looks like the one below
User clicks on the link included in the email
The user needs to verify his/her identity by clicking Send Code
Another email is sent to the recipient with the code. The user needs to use it within 15 minutes
Once the user enters the code and clicks Verify.
Users will be able to:
Download files and subfolders
Upload files and folders
Modify files and subfolders
Create and upload new subfolders
Delete files and subfolders
Move and copy files and subfolders
Rename files and subfolders
Share files and folders with others
Change views (if additional views exist on a document library)
Filter by custom metadata (if custom metadata has been created).
I hope this is helpful!
Akshay.Jadhav