Go to this link Amazon.com: Appstore for Android and enter your mobile number. Should be on right side of page. The website will give instructions on how to do it. Also remember to follow the instructions to get your phone to accept apps from third party sources.
On the page this link takes you to, on the right hand side towards the top is a box titled "Get Started"
You can enter your email or mobile number. I entered my mobile number, then click the link in the txt message you will get. This will download the apk to your "download" folder. Go into that folder and click on that file and then follow the process through to install.