Open the Photos app
Go to the album or view you want to select from
Tap Select in the top-right corner
Tap each photo individually
To select many photos faster, drag your finger across rows of photos
To select all photos in an album, use a third-party app or select items in batches, since Photos does not have a universal Select All button in every view
In some albums or views, tap Select All if it appears
After selecting, tap the Share, Delete, or Add to action you want