Well you would need 3 things. 1. The instructions I am including below. 2. A phone running, or upgradable to, Android "Marshmallow". 3. A phone that the manufacturer hasn't restricted this option on. This last one is the kicker. If you are running the right Android version, but the instructions included don't work for you, then you probably have a phone that was restricted by the manufacturer.
How to turn 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...