The big secret for me was doing this:
1. Run Google Play, also known as Play Store.
2. Click on three lines icon at top left.
3. You should see your current email address. Click on the tiny Arrow icon to the right of the email address.
4. Select your desired Google Account by clicking on its email address in the dropdown box.
5. If you have games that were installed using an incorrect email address, uninstall each app and then install it again. This should allow you to make payments.
<flame>Cell phones make some operations easy and others hard due to lack of standardized interfaces. In some ways, cell phones are just as difficult to use as desktop computers. <end of flame>