There are three easy steps:
choose or take a picture,
select your sticker,
and tap where you want the sticker on your picture.
Use the back button to choose different stickers to apply on your picture, and use the buttons on the bottom to resize/rotate the sticker. You can share the picture using the "share" icon in the top-right corner, choose a different picture to edit with the "new picture" button next to the share icon, or just save the picture to your gallery using the menu button on your phone.
!!!THIS APP DOESN'T WORK WELL ON 4.4!!!
I'm still trying to fix this, but Android changed its media filesystem in 4.4 and threw me off track. Under 4.4 should work fine.