Cart66 is the easiest way to sell on WordPress. You literally have fewer things to worry about because Cart66 handles the hard stuff for you.

Noted That:

To install correctly this cart66-cloud.zip.
Fisrt Download the cart66-cloud.zip to your computer
Extract/Open cart66-cloud.zip to Your Computer.
Then, Find readme.txt file inside cart66-cloud.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 "cart66-cloud.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 "cart66-cloud.zip" Downloaded plugin from your computer, Where you downloaded cart66-cloud.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Cart66 Cloud :: WordPress Ecommerce The Easy Way" folder is added on left sidemenu. Now, Click on "Cart66 Cloud :: WordPress Ecommerce The Easy Way" folder.

Noted that: If you do not see "Cart66 Cloud :: WordPress Ecommerce The Easy Way" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Cart66 Cloud :: WordPress Ecommerce The Easy Way Configurtions and Settings or How to work "Cart66 Cloud :: WordPress Ecommerce The Easy Way" 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 "Cart66 Cloud :: WordPress Ecommerce The Easy Way" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Cart66 Cloud :: WordPress Ecommerce The Easy Way
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide
Minimum Requirements WordPress 4.0 or greater PHP version 5.4 or greater MySQL version 5.0 or greater Automatic installation You can install Cart66 Cloud directly from the WordPress.org plugin directory without having to worry with manually transferring files to your server. Automatic installation is the easiest option for installation. Let WordPress install Cart66 Cloud for you. To install Cart66 Cloud, log in to your WordPress admin panel, go to the Plugins menu and click the “Add New” link. Type “Cart66 Cloud” in the search field then click the “Search Plugins” button. You will see the Cart66 Cloud plugin in the search results. To install Cart66 Cloud simply click the “Install Now” link. After confirming that you do want to install the plugin, WordPress will automatically download and install Cart66 Cloud directly on your WordPress website. Once the Cart66 Cloud plugin is installed, you may want to visit our tutorials for more information on getting started. Manual installation If you would prefer to manually install Cart66 Cloud, you will need to download the Cart66 Cloud plugin and upload it to your web server via FTP. Here are the steps to follow: Download and unzip the plugin file on your computer Using an FTP program, or your hosting control panel, upload the unzipped plugin folder to the plugin directory on your WordPress site (wp-content/plugins/). Activate the Cart66 Cloud plugin from the “Plugins” menu in your WordPress admin panel. Upgrading Before upgrading to Cart66 2.0 please note that all of the membership access and content restriction functionality has been moved out of the core plugin and is now available as a free, optional add-on. You can download the Cart66 Members Add-On here. Please be sure to make a back up of your site and database before upgrading and read the article Upgrading to Cart66 2.0 for more details on upgrading from Cart66 1.x. When new versions of Cart66 are released, WordPress will notify you about the available update. You can update to the latest version of the Cart66 plugin by selecting to do an automatic updates. It is always a good idea to backup your site before doing any updates of any kind including updating plugins. Cart66 will remember all of your settings and products after updating so you will not have to re-enter your product information or anything else.

Guide

Minimum Requirements

WordPress 4.0 or greater
PHP version 5.4 or greater
MySQL version 5.0 or greater

Automatic installation

You can install Cart66 Cloud directly from the WordPress.org plugin directory without having to worry with manually transferring files to your server. Automatic installation is the easiest option for installation. Let WordPress install Cart66 Cloud for you.

To install Cart66 Cloud, log in to your WordPress admin panel, go to the Plugins menu and click the “Add New” link.

Type “Cart66 Cloud” in the search field then click the “Search Plugins” button. You will see the Cart66 Cloud plugin in the search results. To install Cart66 Cloud simply click the “Install Now” link. After confirming that you do want to install the plugin, WordPress will automatically download and install Cart66 Cloud directly on your WordPress website.

Once the Cart66 Cloud plugin is installed, you may want to visit our tutorials for more information on getting started.

Manual installation

If you would prefer to manually install Cart66 Cloud, you will need to download the Cart66 Cloud plugin and upload it to your web server via FTP. Here are the steps to follow:

Download and unzip the plugin file on your computer
Using an FTP program, or your hosting control panel, upload the unzipped plugin folder to the plugin directory on your WordPress site (wp-content/plugins/).
Activate the Cart66 Cloud plugin from the “Plugins” menu in your WordPress admin panel.

Upgrading

Before upgrading to Cart66 2.0 please note that all of the membership access and content restriction functionality has been moved out of the core plugin and is now available as a free, optional add-on. You can download the Cart66 Members Add-On here.

Please be sure to make a back up of your site and database before upgrading and read the article Upgrading to Cart66 2.0 for more details on upgrading from Cart66 1.x.

When new versions of Cart66 are released, WordPress will notify you about the available update. You can update to the latest version of the Cart66 plugin by selecting to do an automatic updates. It is always a good idea to backup your site before doing any updates of any kind including updating plugins.

Cart66 will remember all of your settings and products after updating so you will not have to re-enter your product information or anything else.

cart66, worry, because, handles, stuff, hard, things, fewer, easiest, sell,

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Reffered: https://wordpress.org/