First, you need to download the plugin from the source (which will be a zip file).
Next, you need to go to the WordPress admin area and visit Plugins
» Add New page.
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After that, click on the ‘Upload Plugin’ button on top of the page.
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This will reveal the plugin upload form. Here you need to click on the ‘Choose File’ button and select the plugin file you downloaded earlier to your computer.
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After you have selected the file, you need to click on the ‘Install Now’ button.
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WordPress will now upload the plugin file from your computer and install it for you. You will see a success message like this after the installation is finished.
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Once installed, you need to click on the Activate Plugin link to start using the plugin.
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After activation, you need to enable plugin & configure payment options like Merchant ID, API key provided by payment options.