If you're after completing this achievement ASAP, open the spoiler. If you are after enjoying the story without spoiler, just continue playing as you would and this will eventually unlock.
*** Spoiler - click to reveal ***
Head over to the cabin. Inside, examine the bookshelf and the armor stand. You'll go through some dialogue with Petra about your friendship. Afterwards, change the beds in their correct order (Purple, blue, red). Then get the book from behind the purple bed. Again, you'll have a conversation with Petra. You'll also discuss Romeo, Xara and Fred.
You'll place fireworks to meet up with the rest. The enderman will attack again. Follow the QTE's. Then fight off some spiders and zombies. Afterwards, mine some stone and cobblestone and rebuild a part of the house, so that the portal will work. You get to choose between taking Radar or taking Fred's people with you.