The process of customizing your snap code by adding selfies is actually incredibly simple. First you are going to want to open the Snapchat app. Next you will be sent to the camera page of the app by default. There will be a ghost symbol at the top of the page in the center.
Tap the ghost at the top center of the screen, Or just swipe from top to bottom across the screen. You will be presented with this screen:
Your snap code is the yellow box above your username. Your snap code can be snapped by other snappers and it automatically adds you to their friends. To customize your snap code with a selfie, simply tap on your snap code. When you tap on your snap code it will blow up and the white ghost will turn into your viewfinder for capturing your selfies.
To capture the selfies you just tap the circle underneath your code. The selfies will be captured in a burst of four pictures with a few seconds in between so that you can make another funny face or just adjust. Before the pictures are taken it will say, Ready, Set, Go. After the pictures are taken, they are turned into a GIF and will play in the silhouette of the ghost. When you request to add people they will be able to see your selfie GIF. If you want to erase your customized snap code and return it to the default white ghost, just tap on your code again and tap the top left button that looks like a ghost with an arrow rotating counter clockwise around it.