All good guidance so far.
Chinese elms are a subtropical species, so they can survive indoors, but you need to give them ALLOT of light. More than a window, like a window and a grow bulb.
Watering; try the chopstick trick. Jab a wooden/bamboo chopstick or skewer into the soil all the way to the bottom of the pot, and leave it there. Use it like a dipstick to check soil moisture, and water when it's all but dry.