Depends on what 'stuff' you are trying to get to go on the SD card. If it is just photos, music, etc., (not apps) then you will need to locate the phones file manager (if it doesn't have one you will need to download a file management app) and use that to move the files over to the card. Or you can connect the phone to a computer, via USB cord. Move the files over to an empty folder on the computer, then move them back (still through the USB cord) to the SD card. This second option is much faster. Then you would need to access certain apps, like the camera app, and go into its settings to change the place new photos are saved to as the the SD card. The same would be done with whatever app you download files through (again except apps themselves).
NOTE: If you are wanting to make apps go to the card, and run from it, then that is a whole other story. You would need to make that change (if possible on your device) before doing any of the above.
First, you need to be using a phone that is running at least Android "Marshmallow" (or newer) 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 phone, or they don't work for you, then you have a phone that either has an older Android version or has been 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 phone.
NOTE: If you don't reboot the phone, 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 phone, some things may not work correctly, so make sure you do the reboot.