The following steps require you to login to your Nextcloud account via https://share.mayfirst.org/.
Navigate to the folder in which you would like to place your shared folder.
Click the "+ New" button to create the folder.
With the folder to be shared displayed (in this example it is called "My Public Shared Folder"), click the "..." menu and select "Open Details":

Click the "Sharing" tab to generate a share link and make the folder accessible. The link will be automatically copied to your clipboard so you can just paste it.

You can always come back later and click on the icon with the up/down arrow to re-copy the link.

Optionally, you may click on the "..." menu by the share menu and unselect "Set Expiration Date" so the folder is shared forever.

Note: be sure to click the X in the top right corner to close this share window.
Now, you can click to open the folder you create and add files to it. Do not add a share link to any of these files. Do not click the Share option for any of these files (because that will automatically create a share link).
All files in your folder will all be accessible because their enclosing folder has been shared.