Here is a text walkthrough of Shadowgate that walks you through the game to get all six achievements.
Note 1: There is a lit torch in the upper right above your inventory. If this goes out, you'll die. Because of this, you have two options:
1) Pick up torches from the walls along the way as you play. Whenever the music changes, it means time is running out on your torch. USE the lit torch on a torch in your inventory to stay lit and not die.
2) Ignore this aspect of this game completely. After you die, you can continue and you'll be around where you were when you died, but your torch will be fully lit again and you won't have to use any extra torches. You will die in the middle of doing various actions, but you won't lose much time and won't have to deal with collecting torches and keeping them lit. I personally did this method.
Note 2: I am only going to tell you where to MOVE and have omitted the opening of doors to keep it brief. If there is a door where I tell you to go, obviously open it first then move.
You can do this anytime during this playthrough, but to get it out of the way, start by selecting USE and then using the lit torch in the upper right on Self in the lower right. Do this three times to set yourself on fire, die, and unlock "A Danger to Yourself."
Continue, and this time OPEN the skull above the door and TAKE the Key1. MOVE through the door now. Inside, you can TAKE the torches from the wall if you want to keep your torch lit all game, otherwise ignore them. USE Key1 on the brown door in the back on the left and MOVE through it. Two more torches are available on the walls to TAKE. OPEN the book and TAKE Key2. MOVE back and USE Key2 on the grey door on the right and MOVE through it. TAKE the Sword and Sling and then leave. MOVE through the left door again where the book was, and HIT the white stone on the left side of the back wall. This will open a hole. MOVE through it. TAKE the Arrow right in front of you, and then USE the torch on the left wall to open a door. MOVE through it, then to the top left square and you'll be confronted by a monster. To defeat it, you have to USE your lit torch on the Torch in your inventory (note that this Torch is separate from your stock of Torch=X and should be right below that). This will defeat the monster. TAKE the Blue Cloak hanging, and the torches on the walls if you want them, then MOVE through the door. There is another Torch on the wall if you want to TAKE it. LOOK at the letters EPOR on the right wall twice to learn a spell, then USE that spell. MOVE up to be in a room of mirrors. TAKE the Broom on the left, the torches if you want them, then MOVE down to be in a room of coffins.
OPEN the coffin on the right closest to you, then USE your burning torch on the mummy and TAKE the Scepter he drops. Now OPEN the next coffin on the right, OPEN the bag that falls out, and TAKE the three Coppcoins. MOVE down and then through the door on the right, then forward one more time to be at a waterfall. TAKE one of the Stones on the ground, then MOVE behind the waterfall. HIT the protruding rock in front of you, OPEN the bag, and TAKE all three different colored Gems. MOVE down three times and this time go through the left door. USE Whitegem on the hole next to the door, then TAKE the Sphere. There are also two more torches available in here. MOVE through the door to be in a tunnel with a bunch of objects on the ground in front of you. TAKE the Shield first and the dragon will start blowing fire at you. You need to TAKE three more items from the ground (any 3) and the dragon will melt through your shield, killing you and unlocking "Melting Point."
Continue and MOVE forward into the dragon tunnel and again TAKE the Shield first. This time, TAKE only the Spear and Hammer, then MOVE back twice and then through the right door. USE the Sphere on the water to freeze it, USE the burning torch on the ice to free up the Sphere and then TAKE it back, and also TAKE Key3 from the skeleton. MOVE down and then up twice and you should be back in the mirror room. USE the Hammer on the right mirror to die and unlock "And beyond."
Continue, and this time USE the Hammer on the middle mirror and then USE Key3 on the door behind it. USE the Cloak on Self (lower right) and MOVE through the door. USE the Sphere on the fire to get rid of it, then MOVE forward twice and a troll will pop up and ask for gold. USE two Coppcoins on him to make him mad, pull the bridge, and kill you, unlocking "Trolled."
Continue and MOVE up twice to be back at the troll. This time USE the Spear on him to get rid of him. MOVE forward to encounter a cyclops. USE the Stone on the Sling, then USE the Sling on the cyclops to knock him down, and then USE the Sword on the cyclops to kill it. USE the crank on the well on the right to raise the bucket, then OPEN the bucket and TAKE the Gauntlet. MOVE forward to find another torch on the wall if you want it, then go into the lower left room. OPEN the desk, and then OPEN Scroll3 and Scroll4 to learn two more spells. TAKE Key5 and Glasses. USE Glasses on Self and then OPEN the book on the desk to learn another spell. TAKE the Map on the wall on the right and the Skull on the shelf on the left. USE Redgem on the hole by the bookcase to open it, and MOVE through. TAKE the Bellows hanging on the fireplace on the right, and then USE the spell Terrakk to open the globe, then TAKE Key6 from inside. MOVE down twice and then through the top left door. USE the white stone on the ground to make a bottle of water appear. TAKE the Water, as well as the Horseshoe above the doorway on the right, then MOVE through that doorway. USE the Gauntlet on Self, then TAKE the Flute. USE the Flute, then TAKE the Ring from the tree on the left. MOVE back twice and then into the top door. You should be in a room with three doors. USE your burning torch on the carpet and then TAKE Key4 from the ground. Also TAKE the Mirror from the wall. USE Key5 on the upper right door and MOVE through. MOVE forward and you'll encounter a Sphinx that will present you with one of these riddles:
Q: I'm fire's best friend, my body swells with wind, with my nose I blow, how the embers glow.
Q: I've no eyes, but once I did see, thoughts had I, but now I'm white and empty.
Q: First burnt and beaten, then drowned and pierced, then stepped on by long-faced animals.
Q: It has towns but no houses, forests but no trees, rivers but no fish.
Q: You look at me, I look back, your right hand raises, I my left, you speak, but I in vain.
Q: Long neck, no hands, 100 legs, cannot stand, born of forest nest, against a wall I rest.
Whichever riddle you get, USE the item that is the answer on the Sphinx and he'll let you pass, so MOVE up the stairs. TAKE the Star from the poster on the back left, then MOVE forward to find a woman on the ground. TAKE the green object on the right to be killed by her and unlock "Damsel in Distress."
Continue, and MOVE down twice to be back in the 3-door room. USE Key6 on the upper left door then MOVE through it, and MOVE forward again to have a hellhound appear. USE the Water on him to get rid of him, then TAKE the Horn on the ground. MOVE forward and you'll be confronted by a dragon. USE the Star on him to destroy him, then TAKE the Talisman. MOVE back twice to be back in the 3-door room. USE Key4 on the bottom door and MOVE through it. There are two more torches here if you want them, otherwise MOVE through the left door and then left again to find some items on the ground. OPEN the bag and TAKE the Big Coin and one Gold Coin. MOVE back twice and this time go through the right door to find a skeleton on a throne. USE the Scepter on the skeleton's left hand to open up the wall, then USE the Ring on the hole to move the throne. MOVE through the opening, TAKE the torches if you want them, then MOVE forward again to find two gargoyles. USE the spell Illumina to stun the gargoyles, then MOVE to the opening on the back left. USE the handle on the well to open it, then USE the Big Coin on the well to open it up. MOVE forward (into the well) to be in some ethereal area. USE the Mallet hanging on the side of the gong on the gong itself to summon the Ferryman. USE your Gold Coin on him and then MOVE to the raft. USE the Talisman on the left slot and then USE the Horn. MOVE through the opening and you'll find a person in blue and a dragon. USE the Sword on the person in blue to be killed and unlock "Almost There."