It depends on what you are wanting to download to the card.
If you are talking music, videos, photos, etc. (not apps) then you just need to change the settings in whatever app you are downloading them through to save to the card rather than the tablet.
However, if you are trying to download apps directly to the card, that requires a bit more. 1. You will need to have a device that is running at least Android "Marshmallow" or newer. 2. The device must not have had this option restricted by the manufacturer (which does happen often). And 3. The instructions I am including below.
NOTE: If any of the steps are missing for you, or don't work for you, and you are using the right Android version, then the device had this restricted by the manufacturer.
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 “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.
How to turn the "External" SD card into "Internal" storage on "Nougat".
IMPORTANT NOTE: Be sure you start with a blank SD card. When the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device), so anything on it prior would be gone.
1. Go to "Settings", and then select "Storage & USB".
2. At the bottom of the list you should see the SD card's details, including the option to format it and make it "Internal" storage.
3. Once this is done, reboot the device and you can start running things from the card.
If you don't reboot the tablet, some things may not work correctly, so make sure you do the reboot.