Breeding horses is not as simple as getting a stallion to mount a mare. Breeding a stallion can be a very dangerous activity so if you are not very experienced handling horses, and specifically stallions, breeding is best left to professionals.
Stallions cannot be forced to mount a mare. You cannot get a stallion to mount a mare if the mare is not at the appropriate point in her cycle- something a stallion can sense. Trying to force a stallion to mount a mare who is not receptive is an easy way to seriously injure yourself, your stallion, or the mare. If you are unable to tell when the mare becomes receptive to the stallion and ready to be mounted, speak to your vet for more information on safe and responsible horse breeding.