If you are just trying to move files (music, photos, videos, etc.,) and not apps, then you just need to either move them one at a time to the card, or connect the tablet to a computer, via USB cord, then move the files over to a folder on the computer, then copy them back over to the card via the same USB connection.
Now if you are trying to move apps, it will depend on two things. 1. If the tablet is running, or can be upgraded to, Android "Marshmallow" and 2. As long as the manufacturer hasn't restricted this option on the device, which does happen. Here are the instructions on how to make the change.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs...