Expand the "Select available content" section, and choose what expansions you own. This will set what adversaries you can select in the next section.
Expand the "Select Adversary" section. Choose your adversary, or multiples. You can use the button for selecting a random adversary. Use the slider below the adversary to choose the level. The difficulty rating will automatically adjust - this is an estimate only when using multiple adversaries.
The remaining sections should assist you in your play of Spirit Island. Setup will show the game setup steps, reflecting any changes for the selected adversaries and expansions. Loss Rules will inform you if there are any additional ways to trigger a loss. The Game Rounds section shows you the health and damage of adversary units and dahan. Any game rounds that are modified by your adversary are highlighted in red, to make it easier to see and remember what's different. Clicking or tapping on the title of a round will give an explanation of that round, including changes due to your selected adversary.