Couple of suggestions:
Use the front facing camera (not the selfie camera). The video quality will be much better.
Find an uncluttered background to use.
Make sure the room is quiet (no TVs, radios, or other extraneous noises, or people talking in the background, no loud appliances in the area). A constant background sound (AC hum, etc.) is better than one that starts and stops, or changes.
Try to stay within 2-3 feet of the phone (less if the video framing allows it) so the phone’s microphone has a decent chance of cleanly picking up your voice.
Don’t move your head or body around too much as you tape. Look at your phone, imagine that it’s your best friend, and talk to that friend directly.
If you feel that you’ve made a mistake and need to re-tape something, don’t stop the recording, just calmly go back to the beginning of your sentence or thought and re-record it. Even professionals almost never tape their entire videos in one take.
The trick is to go back quickly and to try to keep your focus, speaking pace, the tone of your voice , and the energy of your delivery the same as it was. Also, try to keep yourself in the same position for the repeat, so the two pieces don’t look strange when edited together.
Hope these help,