To get to the Desert Colossus :

     Go to the gate near the entrance to the desert. Climb up the ladder and talk to the gate keeper. She will raise the gate. Go through the gate. Now walk into the desert until you get to a river of sand. Equip your hover boots, and then cross it. Follow the red flags until you get to a sign. Turn left and follow the path until you get to a guy on his carpet.
     He will try to sell you bombuchu's. Don't buy them. Keep following the flags until you get to a stone structure. Walk up to the top and use the lens of truth to see the ghost. This ghost will lead the way to Desert Colossus. Keep following it until it brings you to the entrance of the Desert Colossus.

What to do at the Desert Colossus :

     Walk until you get to the entrance of the Spirit Temple. Go inside it. There is nothing to do in there, so walk back out to meet Shiek. Shiek will teach you the Requim of the Spirit. This will allow you to warp to the Desert Colossus any time.
     Now you must go back in time. Use the Prelude of Light to warp to the Temple of Time. Place the Master Sword into it's holder. Now, as young Link, play the Requim of the Spirit to warp to the Desert Colossus. Now enter the Spirit Temple.