1. Use a solar filter. To take pictures during the eclipse, you need to cover your phone's lens with either a solar filter or your eclipse glasses. Regular sunglasses will NOT work. And only take the filter off if you're lucky enough to be in the path of totality, when the sun is completely covered.
2. Use a tripod to keep your phone stable. To minimize any small movements made by your hands.
3. Take pictures of stuff around you. If you don't have a solar filter, eclipse glasses, or a tripod, take pictures of the stuff going on around you during the eclipse. Things like pets, and other people looking up at the sky.
4. Practice by taking photos just after sunset to get an idea of what the light levels will be like during the eclipse.