Turns 1-4: play Swamps.

Turn 5: Play a Swamp, then play Sidisi, exploiting herself to get Ad Nauseam. Then discard a Swamp at the end of the turn because you have 8 cards in your hand.

Turn 6: Do not play a land yet. Pray to Emrakul that your opponents don't have a counterspell, then play Ad Nauseam, drawing your entire deck, taking, at most, 5 damage. Then play Glacial Chasm as your land for the turn, sacrificing a Swamp. Play Lotus Petal, then crack it for  to cast Dark Ritual, floating . Use one mana to play Elixir of Immortality as a contingency plan. Then cast Sickening Dreams, discarding your entire hand except three Swamps, dealing 84 damage to each opponent, with Glacial Chasm preventing the damage that would normally be dealt to you, thus probably winning you the game.

If you successfully resolved Ad Nauseam: If you have 21 or more life, keep Glacial Chasm, otherwise let it die. Then crack Elixir of Immortality on your turn 7 upkeep (after the Glacial Chasmtrigger) to reset the combo and avoid decking out. Then on turn 9 play Sidisi to get Ad Nauseam and start the combo again on turn 10.

If you did not resolve Ad Nauseam: Play Sidisi on turn 7 to get Codex Shredder. On turn 8 play Codex Shredder and sac it to get Ad Nauseam back. On turn 9, start the combo again.

You can try Nakhla's excellent primer, Sidisi's Reign Of Filth, which is blazing fast cEDH combo deck capable of winning consistently by turn 3, with turn 2 wins being common. And, unlike this deck, it can actually win against the same person more than once.

If you don't want to drop $2k on Nakhla's list you can try my $130 Sidisi deck, Sidisi Ad Nauseam Fishbowl. It is not nearly as fast or resilient as Nakhla's list, but it does follow a similar game plan, winning by storming off with Aetherflux Reservoir, or by casting a giant Exsanguinate.