Assuming your foal is healthy, has been eating feed and hay, is four to six months old and showing signs of independence, he or she is ready to be weaned.
There are several methods, depending on whether you are dealing with a large group of horses or a single mare and foal. The traditional method is to move the mare(s) into a distant paddock so that the foal, or foals, cannot see or hear them.
PROS: Foals remain with their buddies in a familiar pasture and after a couple of days will realize they will not perish without their mothers.
CONS: This generally results in a lot of whinnying and running, which is stressful for both the foal (and the owner) and carries the potential for injury. This is not recommended for weaning a single foal, unless he has some trusted, gentle pasture-mates to keep him company.
A better method is to remove one or two mares each day, starting with the dams of the oldest foal(s).
PROS: A foal will appear to be a bit happier if they can still see adult horses, and the gradual population change keeps them calmer.
CONS: Foals may be nipped or kicked if they bother cranky mares while searching for their own dams.
Stall confinement involves removing the mare and keeping the foal inside for a couple of days before turning him out.
PROS: Works if paddocks are too muddy, etc., for safe turnout.
CONS: Foals become quite distressed, especially if they cannot see any other horses, and may injure themselves trying to get out of the stall.
The best method is fence-line weaning, where mares and foals are separated by a strong fence that is constructed in a way to prevent nursing.
PROS: Foals can still see, hear, and smell their mothers, who can be removed with little fuss in about a week. A Texas A&M University study found that cortisol (stress hormone) levels in foals fence-line weaned were lower than foals weaned the traditional way. Less whinnying and frantic galloping was also observed. Good method for smaller properties with only a couple of paddocks.
CONS: None, as long as the fencing is safe and the foals are monitored often, as they are extremely adept at getting heads and legs stuck in the most unlikely places.