Warp to the Bonfire in Anor Londo, then head out and left.
Then, head towards the building in front of you and take the first right, fight the giant there and head up the stairs. Here you will find the Duke's Archives, in addition to a boar. Take this bor out with Pyromancies and proceed forward, around the corner will be another Boar. Then, you will reach the first bonfire.
Head up the lift and into the heart of the Archives. Immediately you will be attacked by multiple crystal soldiers, use magic or ranged options to take them out one by one, with the archer going first, then you will see a Crystal Giant to your right, take this out and then go towards the main room and kill the channeler. To it's left will be another archer, take this out and head up the stairs to take out a few more of these crystal Hollows.
Head left at the top of the stairs and run to the end to get a Twinkling Titanite from the chest. Then head to the right side of the staircase.
Immediately to your left will be an exit with 2 Hollows guarding some Titanite, grab it and head left. Kill the two crystal Hollows and attack the chest at the end to start fighting a Mimic who will drop a Crystal Knight's Shield. Then, head all the way up and towards the elevator at the top. Take the elevator to the top and start heading into the path you see on the right. You'll fight a Crystal Knight at the top here, and then encounter a fog gate. Pass through.
You will be immediately killed by Seath, but fret not, you will respawn deeper inside. Light the bonfire to your right and then kill the Snake Guard for the key to get out of the cell. After a short cutscene, it becomes clear that you've got to fight your way down. Open the door to te right and jump down to get an Archive Extra Key. then keep heading down. Then, climb up the ladder to the left of the archway.
Kill the 3 tough snake enemies at the top and then head left towards a chest containing the Giant Door Key. Then, pull the lever to stop the music. Head all the way up the stairs again and up the ladder towards the Giant Door.
If you're looking for more help, our Dark Souls walkthrough and guide can help with all other areas of the game, including the dreaded Taurus Demon, Capra Demon, Ornstein and Smough bosses. Meanwhile, we have explainers on how to access DLC Artorias of the Abyss, the best gift and best class in Dark Souls, Dark Souls' best weapons, Dark Souls Covenants, Dark Souls Estus Flasks locations, Dark Souls Titanite Slab locations and NPC quests including Lautrec of Carim.
Then, head out and to the right will be a Crystal Hollow. Kill this and then you will spot the archer at the end of the doorway. Head towards there and to the left will be an additional Crystal Hollow. Then, head in the doorway to the right and kill the channeler to your left.
Go down the path with the channeler and past the archway will be a crystal hollow to your right. Then, kill the archer ahead and head up the staircase and rotate it. Head up and deal with the archer, then follow the path left and go through the archway.
Through the archway, dispatch the crystal hollow and drop down to the staircase you see below. Kill the channeler at the top of the stairs, then turn the staircase. Head into the archway, and deal with the sniper ahead. Climb down the stairwell and pull the lever ahead.
Head out through the arch that the stairwell opened and head left to get to the next Bonfire. Then, head back to the room with the lever and forward into the next room, on the left will be a chest with a Giant Cell Key.
On the right is a mimic, and then head around the bookcase to find the Crystal Ember. You will also find a chest with the channelers set in this room. After some looting, Pull the lever at the end to reveal a hidden staircase. Head down and behind you will be a Crystal Hollow, take care of this and walk through the Fog Gate.
Head down the ladder and deal with the Crystal Giants ahead. Then, head down the slope and onto the crystal path. Welcome to the Crystal Caves.