Well if you are trying to download videos, music, photos, etc. to the card, you just have to change the settings in the app you are doing the downloading through to save to the card rather than the device. Unless the videos are say from Netflix, which won't download to a card at all. It is restricted.
If, however, you are trying to do this with apps themselves, that isn't possible without making changes to the device and card. The following is how, if it will work on that device (doesn't work for all).
First, you need to be using a phone 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...