First and foremost: The goal of this deck is to kill your opponent with a 12/12 spider with rush.
Second: You will not succeed most of the time, if your opponent's deck is any good. This is a casual meme deck.
As stated above, rush spider combo. If you lose any of the pieces, then you just play big bodies like Gemcutter Spider, Tangleskein, and Herald of Conquest straight up, and give them overrun. Until then, you chump block things while you burn all 6 colors and collect your combo pieces.
As a rainbow-colored deck, burning wisely is incredibly important. To play every card in your deck, you need to end up burning BBYYRGOOPP exactly. Your burn priority for the first few turns should be OPYBBGR. This is a guideline, not an absolute need, since in 6-color decks, your ability to have the right color to burn is quite limited. Aside from blue, most of your double-gem cards are supposed to be played later into the game, so you can prioritize burning second gems later.
The aim of the combo is simply to have a 14/12 with rush hit your opponent in the face. Before your combo turn, you want to have Gemcutter Spider and Freki Sidecar in hand, and Demolition Speedway on the board. Pseudonomicon can be used to fetch your combo pieces, but make sure you have enough spare mana and extra cards in hand to be able to use it. The simple combo would be to play Gemcutter Spider on Speedway, then Sidecar next to it, and hit your opponent for a lot of damage all at once. If you have 10 mana, you can add Allfather's Horn for a more powerful version of the combo - place Spider on Speedway, play Allfather's Horn, then play Sidecar next to it, which lets Spider attack twice, for about 28 damage straight from hand. Herald of Conquest is also a good post-Horn buff, especially if you're using minions other than Spider, or Spider is not on a Speedway.
This deck's mana curve supports playing all 5 Heralds - it already contains 2 of them. You can replace Fated Firebird, Gama Sennin, and Tangleskein with Herald of Famine, Herald of Death, and Herald of War. Be warned - this will upset the burn patterns and make the deck harder to play!
September 21, 2022: -1 Allfather's Horn, -2 Wry Trickster, +1 Desertification Engine, +1 Mandatory Vigor, +1 Racer in Shadow
The Allfather's Horn cost nerf makes it unplayable in the deck, so its role as a combo piece is replaced by Mandatory Vigor. Wry Trickster is harder to play early with a low gem count, so Racer and Desertification Engine are added to ward off early aggro.