Prerequisites
Turn on the forehand can only be of good quality if the horse is on the bit while maintaining clear walk steps in the 'lead up' to the halt that precedes the turn. In the halt he must be attentive, as if anticipating the rider's next commands. The only exception to this rule is for corrective purposes or if the horse is ready for collection.
If the horse understands the leg-yield, a review of that movement can be helpful in reminding him to move away from leg pressure, before he begins the turn on the forehand.
In an indoor school, the rider should choose a suitable place to execute the turn that is clear obstructions and not too close to a wall, as this will influence will hinder the horse as he makes the turn.
Read more about this topic: Turn On The Forehand