Give the first mask to one of the guards in Kakakiro
The second ones goes to a guy in Lost Woods
The third one you shall give to the boy on Kakakiro´s Graveyard
The fourth mask is going to the runner on Hyrule field. Follow him untill nightfall, thats when he will stop.
Go get the Mask of Truth in the Happy Mask Shop

In order to use the Mask of Thruth, put it on and then talk to a stone with the same inscript as on the Door of Time.