Turn off lights in the room and use a flashlight to spot the mosquito
Close doors and windows to keep it from escaping
Use a fly swatter or rolled-up magazine to kill it
Spray an indoor insecticide labeled for mosquitoes, following the label directions
Use a vacuum cleaner to suck it up if it is on a wall or ceiling
Set up a mosquito trap or electric bug zapper indoors
Use a fan to make it harder for the mosquito to fly
Remove standing water in the house to prevent more mosquitoes
Check curtains, corners, and under furniture where it may hide
Wear long sleeves and use mosquito repellent if needed while searching