The 4 items needed to create the dragon platebody: Wish bank would put this in his guide
Item Price
Ruined dragon armour slice Coins 3,900,000
Ruined dragon armour lump Coins 1,200,000
Ruined dragon armour shard Coins 2,100,000
Blast fusion hammer 1,000,000
Total price 8,200,000
Dragon platebody 8,900,011
Profit: 500,011
Smithing Dragon Platebodies can be very profitable and can provide a decent smithing experience rate. However, it is difficult to buy the lumps, shards, and slices off of the Grand Exchange, and constantly changing prices may ruin any chances of a large profit.

All 3 pieces of dragon armour are only dropped by tormented demons which are found in the former area of the Stone of Jas inside the Ancient Guthix Temple. This is locked off to players who have not completed While Guthix Sleeps

After the completion of WGS, it is possible to unlock the mithril door upstairs near the mithril dragons by using a dragonkin key. This key can be made by using half of the strange key (acquired during the quest While Guthix Sleeps) on a mithril dragon, while both pieces of the key are in the player's inventory. This will fuse the pieces together into the dragonkin key.

Beyond the mithril door are three hanging dragon skulls, rib cages, and a giant orb in the centre. Using fire wave on each head will cause a flame to appear in their mouths and they will be moved towards the orb in the centre.

Once the orb is lit, the player can go back in the Ancient Cavern towards a corner south-west of the brutal green dragons. Up the stairs, the player will find the Dragon Forge. It is here, using a blast fusion hammer purchased from the Foreman at the Blast Furnace for 1,000,000 coins, that the player can fuse together the three parts of the dragon platebody with 92 smithing, originally 99 smithing at its release. The process yields 2,000 Smithing xp.

A boost may be used such as Dwarven Stout or Dwarven Stout(m). The Assist System may also be used to create the platebody (also, an assisting player can boost his/her smithing level with Dwarven Stout or Spicy Stew).