I've upgraded my plan and my devices are all showing online. Is there something I need to do to make them available to the Over the Air Updates feature? Do I need to add anything to my sketch to allow this functionality?
The 'BasicOTA' example from Arduino OTA should get you started. You will have to have Python installed as well as far as i remember. This tutorial goes over the whole thing in more detail.
Keep in mind that your sketch-size may not be more than 49% of the total memory available. (Flashsize - SPIFFS) since the new sketch gets copied into flash while the old one is still running. Once that is successful, the first 'jump to' gets the first address of the new sketch and then the unit is restarted.
Well there is the possibility to do an update where the ESP initiates like this where you update the firmware with a binary file of your choice, or like this where you can download the file from a specified location (i've haven't used that method myself)
You do have to make sure that you provide the correct binary of course, correct flash-size and SPIFFS etc. but that is the same as with uploading through the IDE. OTA only works if you have a working sketch on the unit that has the updating part integrated.
So the actual paid feature of the IoT Cloud Plans (Over the Air Updates) has no baring to how I can use OTA on a D1 Mini (ESP8266). I figured it would be an integrated feature set of the ArdionoCloud core and be available automatically once a paid subscription was purchased.