Well, you will need to first be running at least Android "Marshmallow" and second you will need to be using a device that the manufacturer hasn't restricted this option on. You would know it is restricted if any of the steps in the instructions below aren't there on your phone or don't work for you.
How to turn the "External" SD card into "Internal" storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select...