To follow Mai to the stolen heirlooms and defeat the ronin without being seen in The Thief in the Ghost of Tsushima follow Mai and the ronin until one of them stops by a flag. Assassinate the ronin.

Keep following Mai and the ronin until two of them stop near a cart. Wait for a short time until one of them leaves, then assassinate the other ronin.

Keep following Mai and the ronin until you get to an area with a well.