In the middle of Episode 2, after a card game, you have the choice to go with Violet or Louis as they go their separate ways. To see this Easter Egg, you will have to go with Violet.

Also, you can get achievements for friendzoning and romancing them. I recommend saving at the start of the card game and go with the other one later on so you can get both without seeing the same scenes over and over again.

Later Clementine and Violet will be looking at stars. To find this one, look up when Violet said she saw a fish constellation. Click on some stars, and they will soon glow and you can connect them and see your old friend. You can see a video guide here: https://www.youtube.com/watch?v=VGEpqQ5PsTM

This one I actually got by accident, early in Episode 4. At one point you will enter a cave after running from zombies, with a stream in front of you. To trigger this, first you must collect both collectibles, a crystal on the left side of the entrance, and a toy in the right.

After that, you will have to check the stream, and a pile of wood next to it, and then you can see the stream again. At this point, you will see the floaty boy.
I think you can freely look around other objects or talk with people (because I did) but you would have to interact with these 5 items in order.

So in short:

1. Pick up crystal collectible
   
2. Pick up toy collectible
   
3. Examine the stream
   
4. Examine the pile of wood
   
5. Examine the stream again

Some people seem to say the collectibles don't matter, but I tried interacting with only the last 3 and it didn't work. After I picked them up, it triggered. Here's a video on how to do it, but remember to pick up the collectibles first: https://www.youtube.com/watch?v=EPyjcUJcbN8

I didn't make the videos, so credit goes to each of the video's creators. I definitely missed the Kenny easter egg, and for the tube zombie which I didn't know at first how to re-trigger, I followed the advice of a Redditor in this post.

Pictures of the collectibles are from Krekster's guide, who helped me immensely during my playthrough to not miss achievements and collectibles. https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1699923402