Step 5-(optional):Cry cuz you got stuck, and have a hard time using a key cuz you are not native English speaker , or just you misspelled the same thing 10 times cuz in your mind it was correct)
Enter the Nexus and click on the "Use Key" icon in the bottom-left. You can only do this if you have progressed through the Eldritch storyline to the point where the MC learns that Nexus keys are a thing