This page is littered with different approaches to the same goal. Pick the video that works best for you! Photopea users should be able to follow these too but I've also put a Photopea-specific version at the end as well.
Here's a reasonably straightforward tutorial on how to create a Double Exposure effect in Photoshop!
There are literally hundreds of these tutorials on YouTube. Just search for "Double Exposure Photoshop".
And here's one done in Photopea:
PROJECT 6: DOUBLE EXPOSURE
For this project, you will need to have a picture taken of yourself in front of a very simple background. I recommend the big white background in the studio. You could also use a photo of someone else, but it's way more fun when it's you!
You will also need a landscape shot of your choosing (that you took or that you found on the Internet).
Follow the tutorial above (or ANOTHER of the many tutorials on YouTube) to create a double exposure effect that shows a landscape scene within the shape of your face!
Happy Photoshopping! Upload your before images as well as your final product (in .jpg form) to Classroom.