The MailChimp plugin allows you to quickly and easily add a signup form for your MailChimp list as a widget on your WordPress 2.8 or higher site.

Noted That:

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

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Easy Mailchimp Optin Form" folder is added on left sidemenu. Now, Click on "Easy Mailchimp Optin Form" folder.

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

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

Guide

Guide
This section describes how to install the plugin and get started using it. Version 2.8+ Unzip our archive and upload the entire `mailchimp` directory to your `/wp-content/plugins/` directory Activate the plugin through the ‘Plugins’ menu in WordPress Go to Settings and look for “MailChimp Setup” in the menu Enter your MailChimp API Key and let the plugin verify it. Select One of your lists to have your visitors subscribe to. Finally, go to Appearance->Widgets and drag the `MailChimp Widget` widget into one of your Widget Areas And you are DONE! Advanced If you have a custom coded sidebar or something else special going on where you can’t simply enable the widget through the WordPress GUI, all you need to do is: If you are using WordPress v2.8 or higher, you can use the short-code: [mailchimp Optin=”1?] Where ever you want it to show up.

This section describes how to install the plugin and get started using it.

Version 2.8+

Unzip our archive and upload the entire mailchimp directory to your /wp-content/plugins/ directory
Activate the plugin through the ‘Plugins’ menu in WordPress
Go to Settings and look for “MailChimp Setup” in the menu
Enter your MailChimp API Key and let the plugin verify it.
Select One of your lists to have your visitors subscribe to.
Finally, go to Appearance->Widgets and drag the MailChimp Widget widget into one of your Widget Areas
And you are DONE!

Advanced

If you have a custom coded sidebar or something else special going on where you can’t simply enable the widget through the WordPress GUI, all you need to do is:

If you are using WordPress v2.8 or higher, you can use the short-code:
[mailchimp Optin=”1?]

Where ever you want it to show up.

your, mailchimp, widget, list, wordpress, site, higher, form, plugin, allows,

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Reffered: https://wordpress.org/