2) Pick an extreme condition from the list. There are 11 categories, so if you cross off one, you could just roll a d10. Now look to the rightmost column, which lists common extremophiles living in environments with that condition. Pick one of those.
3) Develop an intelligent alien species that evolved from your base creature, which still needs a similar environment. To ground the alien culture in biology, specify one prominent feature each of personal, religious, and public life that derives from its environmental requirements.
4) The core conflict is that the environment is becoming less extreme in a way that threatens the alien civilization. A boiling environment might be cooling off, while an alkaline one might be acidifying, etc. Establish how your aliens respond to this (and remember, that should derive from their culture, not what humans would do).
5) Now some humans show up and realize that something is going seriously wrong. Create at least one complex character; you can fill out the cast with one or more stock characters. (You may also find these resources useful in creating alien characters.)
6) At this point, another complication occurs. You can invent one or play with this random plot twist generator.
7) Use your understanding of your aliens' biology and culture, and the humans you've added to the equation, to answer these questions: Will the humans try to help the aliens or take advantage of their predicament, or something else? Will the aliens welcome the humans or attack them, or something else? Will they solve the problem, or fail -- and how? Write the results.