There really is no way to guarantee that it will hatch. Even if you took the egg from her and put it in an incubator, it would only have a 50-75% hatch rate.
I would suggest that you make a soft nest with raised sides so that she can't push the egg out of the nest. If she is already in a hanging structure with a hole in it or some other kind of protected area, you can leave the egg in there. Then be sure that she (or one of the other birds) sits on the egg the majority of the time to keep the temp up.
Around day 7-10, you can take a small flashlight (smaller than the egg) and "candle" the egg. At this point, you will either see an empty egg or veins and a blob inside. If you see this, you have a baby growing. But even this does not guarantee a hatch.
Now that you have one egg, I bet that you will start to see a lot more!
Let me know if you still have questions.
Not an active Expert