There are two ways to add photos / videos to your album
Register from library
Long tap a photo / video to display the menu
Tap “Add to Album” to display a list of albums
Tap the album you want to add and the addition is complete
Register from the album
Tap an album to open it
Tap “+” at the bottom left to display the photos / videos in the library
Select the photo / video you want to add and tap “Done”
That is all