Subscribe to Comments allows commenters on an entry to subscribe to e-mail notifications for subsequent comments.

Noted That:

To install correctly this subscribe-to-comments.zip.
Fisrt Download the subscribe-to-comments.zip to your computer
Extract/Open subscribe-to-comments.zip to Your Computer.
Then, Find readme.txt file inside subscribe-to-comments.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "subscribe-to-comments.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "subscribe-to-comments.zip" Downloaded plugin from your computer, Where you downloaded subscribe-to-comments.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Subscribe to Comments" folder is added on left sidemenu. Now, Click on "Subscribe to Comments" folder.

Noted that: If you do not see "Subscribe to Comments" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Subscribe to Comments Configurtions and Settings or How to work "Subscribe to Comments" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Subscribe to Comments" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Subscribe to Comments
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
Put subscribe-to-comments.php into [wordpress_dir]/wp-content/plugins/ Go into the WordPress admin interface and activate the plugin Optional: if your WordPress theme doesn’t have the comment_form hook, or if you would like to manually determine where in your comments form the subscribe checkbox appears, enter this where you would like it: Optional: If you would like to enable users to subscribe to comments without having to first leave a comment, place this somewhere in your template, but make sure it is outside the comments form. A good place would be right after the ending tag for the comments form:

Put subscribe-to-comments.php into [wordpress_dir]/wp-content/plugins/
Go into the WordPress admin interface and activate the plugin
Optional: if your WordPress theme doesn’t have the comment_form hook, or if you would like to manually determine where in your comments form the subscribe checkbox appears, enter this where you would like it:
Optional: If you would like to enable users to subscribe to comments without having to first leave a comment, place this somewhere in your template, but make sure it is outside the comments form. A good place would be right after the ending tag for the comments form:

subscribe, comments, notifications, subsequent, mail, commenters, allows, entry,

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Reffered: https://wordpress.org/