First you have to purchase Stickers through the NC Mall. Once you have them, they’ll automatically be added to your “Sticker Book,” which you can access when creating your NeoSignature in the NeoBoard Preferences.
Click on the “Add Sticker” button, select the Sticker you want from your Sticker Book menu, and it will be added to your NeoSignature - they’ll be represented in the text box with a specific code, but that code will convert to an image in the NeoSig preview and on the boards - just like Smilies!