Feed(Aggre)gator merges a group of RSS feeds into a single widgetized list.

Noted That:

To install correctly this feedaggregator.zip.
Fisrt Download the feedaggregator.zip to your computer
Extract/Open feedaggregator.zip to Your Computer.
Then, Find readme.txt file inside feedaggregator.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 "feedaggregator.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 "feedaggregator.zip" Downloaded plugin from your computer, Where you downloaded feedaggregator.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "feedgator" folder is added on left sidemenu. Now, Click on "feedgator" folder.

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

8. Now you configure yourself oR Watch video tutorial below about feedgator Configurtions and Settings or How to work "feedgator" 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 "feedgator" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is feedgator
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
Upload `feed-gator.php` to the `/wp-content/plugins/` directory Activate the plugin through the ‘Plugins’ menu in WordPress Enter RSS feeds to aggregate and manage your display options through the ‘Widgets’ menu in WordPress (optional) If you want to change the name that is displayed for certain authors, you can do so by modifying the $feedgator_item_author_arr in feed-gator.php. See feed-gator.php for more details. (optional) If you want to assign a certain CSS class for list items from certain authors, you can do so by modifying the $feedgator_item_class_arr in feed-gator.php. See feed-gator.php for more details.

Upload feed-gator.php to the /wp-content/plugins/ directory
Activate the plugin through the ‘Plugins’ menu in WordPress
Enter RSS feeds to aggregate and manage your display options through the ‘Widgets’ menu in WordPress
(optional) If you want to change the name that is displayed for certain authors, you can do so by modifying the $feedgator_item_author_arr in feed-gator.php. See feed-gator.php for more details.
(optional) If you want to assign a certain CSS class for list items from certain authors, you can do so by modifying the $feedgator_item_class_arr in feed-gator.php. See feed-gator.php for more details.

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Reffered: https://wordpress.org/