After the damage always. Like if you have 100 HP and your opponent does 20 damage to you that turn and you go down to 80. Then next turn if they do 20 more damage to you, but you use your healer, then you go back up to however much your healer healed.
Also, make sure you don't buy a healer that doesn't over-heal There's a good article on IDB about over-heal and all your healing needs