How do you get the lens of truth?

The lens of truth is inside the well when you're a kid, which is the only time the block is not there. In order to drain the well, you have to talk to the windmill guy as an adult to learn the Song of Storms, then you have to play it to him as a child. About the Shadow Temple, if you already have the Nocturne of Shadow, then you just play it, go inside, and light all of the torches. If not, then you have to go to the graveyard in Kakariko as an adult and Sheik should be there to teach it to you.

You get the eye of truth when you are a young link and you have the song of rain and drain the well in karoke village.

To get to the shadow temple you need the item above then go to the graveyard and run to the very back of the place and get on top of the ledge that is above the graves.

To get inside the Shadow Temple, you need to go to Hyrule Castle and take the root to the right to the Fairy's fountain (You need to bomb it to get inside) to unlock Din's Fire. Use the song to get you to the Shadow Temple you get it by going into Kakariko Village. Stand on the platform in the middle of all the torches and use Din's Fire to get access.
To get inside the well and to get the lens of truth go to the windmill as an adult and talk to the guy in the windmill to learn the song of storms. Then go to the windmill as a kid and play the song of storms to drain the well and gain access to the well as a kid (Can't do it as an adult).