From Wikipedia: In photography, a multiple exposure is the superimposition of two or more individual exposures to create a single photograph. The exposure values may or may not be identical to each other.
So I've been meaning to learn how to do this and while I was working on some pictures, I had accidentally figured it out. Some of you might already know how to make a double exposure in Photoshop but for those that don't, this is pretty easy!
Open both pictures in Photoshop. Make sure they are the same size.
Choose one of the pictures by clicking on it and go to Select > All. (In my case I clicked on the leaf first.)
Next go to Edit > Copy. Now click on the other picture (the wood in my case) and go to Edit > Paste.
Now the leaf photo will be on top of the wood photo.
Go over to Opacity Bar and move it to the left until you like it (circled in red).
The last step is to flatten the image and you're done! Have fun playing around with different photos and I hope this helped you!