Summarize:

You'll be able to use this method with both app for mobile device and http://photos.google.com. It's at the bottom of the mobile app, and on the left side of the website. The icon looks like a square with a bookmark at its upper-right edge. Once clicked or tapped, a list of your albums will appear. If you don't see any albums, you'll need to create one first. The album's current contents will appear. It's at the top right area of the screen and looks like a photo with a plus sign. Now you'll see a list of your photos (outside the album), all containing circles at their upper left corners. When you select a photo, the circle at its upper left corner will turn to a check mark. All photos with check marks will be added to the album. You can select as many photos as you'd like. " It's at the top right corner of the screen. The photos you selected are now added to your album.
Open Google Photos. Click or tap the Albums icon. Click or tap an album to modify. Click or tap the Add Photo icon. Click or tap to select a photo. Click or tap "Done.