Water when the top 50% of the soil is dry. Watering too often to create soggy soil leads to root rot, but letting your poinsettia get bone dry can cause the roots to dry back. Water thoroughly until it flows out of the drainage hole and discard any excess water.
Your poinsettia prefers indirect, but bright light. Too much light will cause the foliage to get crispy and brown. An eastern window is perfect for this plant, or a little further back from a southern or western window works as well.
These tropical plants like consistent warm temperatures. Be sure to keep your poinsettia away from any drafty areas or open vents.
If you notice your poinsettia leaves start to curl and brown at the times, it might need a humidity boost. Try misting, adding a pebble tray, or placing a humidifier nearby.