I am assuming, by what you wrote, that you are wanting to download, install and run apps from a SD card rather than the phone itself? If that is the case, it may or may not be possible from your device. The following is what you would need to be able to make it work.
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 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...