This spoiler free chapter of our guide to The Last of Us 2 contains a walkthrough for the flashback from 3 years ago when Joel and Ellie celebrate the girl's birthday together.
Road to the museum
Follow Joel - he will lead you to a mysterious place. At some point you will both fall into the water. Swim behind the man, diving in the right spot to get some air.
Continue the way forward until you reach the giant dinosaur statue. Climb on it, and when you are at top, jump into the water. Talk to Joel and then enter the museum.
It's a fairly linear location - your job is to approach the objects and interact with them. Examine individual dinosaurs, pick up the hat lying on the bench and then go upstairs.
This section of the museum is very similar to the previous one, except you have to see exhibits related to space travel. Get in the rover, admire the rockets, and lastly get in the spaceship.
Natural History Center
After that, you can leave the museum - go through the door and jump into the water. It turns out that the Natural History Center is closed - have Joel help Ellie get inside through the window. Inside, search the area and move forward, passing through narrow passageways and going deeper into the center. At some point you will run into... a boar.
Joel will join you. Ellie wakes up and the flashback sequence ends.