To make Double Exposure photos with the D3400, we first need to be shooting in RAW format. Go to Menu - Shooting Menu - Image Quality - Hit OK
Set your Image quality to NEF (RAW) + JPEG fine
By doing this when you look at your SD Card, there will be two versions of your photos... One .jpg and one .nef, both the same photo.
Next think about the shots you want to combine. For my first double exposure I will be taking a portrait of someone and a landscape. You cannot see what your images will look like together until after both shots are taken.
Go back into your Menu - Retouch Menu - Image Overlay
Once in the Image Overlay menu, choose your two photos
the x1.0 under each image is the level of transparency the photo will have, lower than 1 will be more transparent, higher than 1 will be more opaque.
Click OK on the Overlay button to preview the image
If you don't like the preview, hit the back button and change the transparency settings.
Continue playing with these settings until you get an image that you like. You may need to retake pictures in order to get an image you enjoy.
Below you can see a variety of portraits with the same landscape background
Carlos in the Hallway
Milos in the classroom
Yasar on black background / Yasar on white background
Turn in a total of 4 double exposure images;
could be a combination of portrait/landscape, portrait/portrait, landscape/landscape, rotation of camera...