Open the Photos app
Select the photo
Press and hold on the subject in the photo
Tap Add Sticker
Open Messages, Notes, or another app that supports stickers
Tap the sticker icon or sticker picker
Select your saved sticker
Drag it into your message or document
To edit it, open the sticker in the sticker drawer and choose an option like Add Effect or Delete
To create a new sticker from another photo, repeat the process