Assuming you would like to use 2Checkout as a payment gateway for your WordPress site, here are instructions on how to add 2Checkout to your WordPress site.
Install and activate the "WooCommerce" plugin. This will add eCommerce functionality to your WordPress site.
Install and activate the "2Checkout Payment Gateway" plugin.
Go to WooCommerce > Settings > Payments and select 2Checkout from the list of payment options.
Configure the plugin settings according to your needs.
Save your changes.
Now when you go to checkout on your WordPress site, you should see 2Checkout as an option.