Always have a reader! This person doesn't have to be an actor. They will be off-camera reading the other character's lines.
Your background should be a blank wall (or something akin to it).
Try to film in a place with no distracting background noises. And be sure that your voice can be heard clearly.
The casting director needs to see your face! A big window to the side or in front of you (off screen) can be a good source of lighting. If you don't have a window, you can purchase ring lights or led panels online. They are easy to set up and use.
Eye line is important. Look at your reader. They should be standing right next to the camera so that we see your face and eyes clearly when you talk to them.
Filming with your phone is fine. Just make sure it's horizontal, not vertical.
Make sure you're framing up your shot so that your upper torso is visible, and your head has just a little breathing room above it (doesn't need to be a lot).
Be sure to slate before your audition! Tape yourself saying your name (and agency if you have one) directly to camera. Make it brief. This is the one time that it's ok to make a cut in your video so that you don't waste time transitioning from your slate to your audition.