The key to cutting crown in a standard miter saw is that the moulding must rest at an angle with the TOP of the crown on the saw's table (imagine the table is the ceiling) and the BOTTOM of the crown leaning against the fence (imagine the fence is the wall.) In essence, everything is upside down. You may want to create a jig that will ensure you keep the moulding at the correct angle each time.
Then, turn your miter box one way or the other (depending on whether you want an inside or outside corner) to 45° and make your cut. Waste a few inches of material to make some "templates" for yourself. Then, when you need to make a similar cut again, it will be easier if you have an example piece to look at.
If you wish to stop a piece short of a corner, try a self return to finish the edge. The miter is the same as it would be on an outside corner, just returned back into the wall.
