Download the Amazon Underground app
Click the apps and games section
Look for an offer for the $5 credit
You have to download one of the 20 apps they have listed
If you have already installed the amazon app on any of your devices, you may be able to do it by installing this Charlie brown app.
You can do it from your web browser by going here and clicking the get app button. You should get a receipt for the app by email
Free $5 Amazon Credit for Android users