This lightweight plugin for WordPress 2.0 or greater gives you the ability to easily allow users to subscribe

Noted That:

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

6. Now, Click on "Active Plugin"

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

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

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

Guide

Guide
Pre-requisites Wordpress 2.x Phplist 2.10.2 or greater. (Download at www.phplist.com) PHP 4.x or 5.x with cURL support Quick Start Instructions Download plugin installation and upload phplist.php and phplist.css to your plug-ins folder, usually /wp-content/plugins Login to wordpress administration panel and activate the plug-in (Under plug-ins). Plugin name is phplist. Navigate to Options -> PHPList and fill in the General Settings configuration as instructed. (Optionally, you can configure the form settings as well – see detailed instructions for information) The last step is to setup the form. If you are adding the subscriber form directly to any post or wordpress page then just add the following comment while inside the wordpress editor: For more advanced users, who are comfortable editing template files, the form can also be added to any template file. For example most users like to display the subscriber form in their sidebar.php. To do this open up the template page (i.e. sidebar.php) and add the following code: `'); echo $content;?>` Detailed instructions available here

Pre-requisites
Wordpress 2.x
Phplist 2.10.2 or greater. (Download at www.phplist.com)
PHP 4.x or 5.x with cURL support

Quick Start Instructions
Download plugin installation and upload phplist.php and phplist.css to your plug-ins folder, usually /wp-content/plugins
Login to wordpress administration panel and activate the plug-in (Under plug-ins). Plugin name is phplist.
Navigate to Options -> PHPList and fill in the General Settings configuration as instructed. (Optionally, you can configure the form settings as well – see detailed instructions for information)
The last step is to setup the form. If you are adding the subscriber form directly to any post or wordpress page then just add the following comment while inside the wordpress editor:

For more advanced users, who are comfortable editing template files, the form can also be added to any template file. For example most users like to display the subscriber form in their sidebar.php. To do this open up the template page (i.e. sidebar.php) and add the following code:

');
echo $content;?>

Detailed instructions available here

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Reffered: https://wordpress.org/