Well if you want to just save photos, videos, music, etc. (not apps) to the sd card, then you just need to change the settings in the app you are downloading through to save to the card rather than the phone.
Now if you are wanting to save Apps to the card that is a whole other story. The following explains...
First, you need to be using a tablet that is running at least Android "Marshmallow" or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your tablet, or they don't work for you, then you have a tablet that either has an older...