First you will need an Amazon AWS account, you can also setup a free trial. This lasts for 12 months. Once you have the account setup follow the steps below.
NOTE It might take a couple hours for your Amazon account to be completely active, as it has to do a credit card verification even though its a free tier.
Amazon Polly is a paid service, so BE AWARE of your usage/expirations.
AWS Free Tier Limits
For Amazon Polly’s Standard voices, the free tier includes 5 million characters per month for speech or Speech Marks requests, for the first 12 months, starting from your first request for speech. For Neural voices, the free tier includes 1 million characters per month for speech or Speech Marks requests, for the first 12 months, starting from your first request for speech.Neural and normal voices are indicated in the voice selections.