Install the plugin
Download it, then in WordPress go to Plugins → Add New → Upload Plugin and activate it.
Create your connection key
In Open Tracked open Settings → CRM and automation and create the key. Copy it, it is shown once.
Connect
In WordPress open Settings → Open Tracked, paste it under Connection key and press Test connection, then Save. Press Send past orders to bring in the last 90 days.
Orders are sent from your server, so none are missed. The plugin also pushes ecommerce events to dataLayer ending in _ot, such as purchase_ot.
Logged-in admins and shop managers are not tracked by default. Test in a private window.