To connect a list in To Do to Base (or another Zenkit product), open the list settings for the list you want to connect: On a mobile device, swipe left on the list name then tap the "..." icon and "Settings". On open lists in the mobile and web/desktop apps, tap or click on the list name.
In the "Zenkit Suite" section of the settings, simply tap or click "Connect" next to "Base". The Zenkit Base app will open on your phone or in your browser and ask whether you want to connect it to Base. Tap or click "Connect" to finish.
If you don't see Base listed in the Zenkit Suite, it means you've not yet installed Base into your account. You can learn more about how to do this under Installing another Zenkit product.
To connect a collection in Base to To Do, open the collection settings, then follow the same steps as above and connect to or install To Do.