1. A concise explanation of the problem you're experiencing.
In the instructions, just before the "Select Cesium World Terrain as your reference terrain in the tiling options" sentence, you could add this sentence: "Answer the 'What kind of data is this' question with 'KML/COLLADA (Tile as 3D Tiles)'."
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
The following sentence tells the user to use a form control that isn't in the UI unless the user previously takes the step described above. The step is guessable, but the instructions are otherwise step-by-step, so including this step seems reasonable.
4. The Cesium version you're using, your operating system and browser.
Version 2020-02-11. OS X 10.13.6. Chrome 80.