I am not an expert, but each year I manage to call a stag or two, but there are no hard and fast rules, it depends upon the stage of the rut, the type of ground, concentration of viable males in the area, and the availability and range of the hind group.
I use a cow horn, but a bit of 32mm poly duct can also serve. Early in the rut, I move around and grunt imitating a stag sorting out his rutting area. When things start going with a bit more gusto I listen for the roars get into a good position and try to make a challenge call, it is important not to be too big and aggressive you can put them off by sounding bigger and more aggressive.
Some will come, some will come for a furtive look and some will disappear without coming for a look. If they are with hinds they will wait for you to make an appearance but may keep up the "duelling bellows".
If you challenge a clearly dominant stag be prepared for a close encounter as he may not be willing to move far unless you directly encroach on his harem and territory. Fix bayonets, I have been ambushed once or twice.
If you are fortunate enough to get a shot take care there may be a another stag in the vicinity, spoiling for fight, I have friends who have been approached by second stags after shooting the first.
A fantastic time of year I love it, the most exciting stalking possible, roaring in big stags in close woodland.
Good luck
H.