Enter or create a game that has the Guardian Angel role enabled.
Enter the game and try to be the first crewmate that dies (Guardian Angel activates for the first crewmate that dies)
Find the impostor and use your best judgement to see who they might kill next
Protect that person
If the impostor tries to kill that person, then you have protected them, so you can enjoy the achievement! If not, however, you can just protect as many people as you can until you protect one that the impostor is trying to kill.