A camera obscura is basically a projection from a round hole. Thats it in its entirety. Light comes through the hole (usually made of metal) and projects in a dark box or room.
So all you need to do is block out all of the light. You can use black plastic or blankets and put them over the windows.
Now get a washer, a piece of cardboard, and tape. Put a hole in the cardboard, and tape the washer on it.
This should be the only source of light in the room. Now hopefully it will project. Sorry for such a rough explanation. My camera does not function in low light also, so I will show you some reference pictures. The larger the washer the brighter but the sharpness is lost. Experiment and have fun!