A former staple deck of modern that slowly has been phased out by the slightly faster Amulet Titan deck gaining more consistency over the years. This deck leans more heavily on Valakut, the Molten Pinnacle than other decks in modern that run the titular land.
The basic idea is to play cards that put lands into play from your deck (or in some cases hand), so that you have 7 lands in play by turn 4. You then play Scapeshift to turn those 7 lands into (generally) one Valakut, and six mountains. This combination of lands entering simultaneously deals 6 triggers of 3 damage for a total of 18 damage. In modern, where we typically assume that a player's starting life total is around 17 from fetching and shocking, is typically enough for lethal damage.
The deck contains a series of ramp cards in Farseek Explore Search for Tomorrow and Sakura-Tribe Elder. Dryad of the Ilysian Grove acts as a sort of super ramp spell. Not only does it allow you play an extra land each turn, it also turns Valakut itself into a mountain, which means that Valakut will trigger with 6 lands in play, since it counts itself. You can theoretically cast Scapeshift with 6 lands in play, grab 4 Valakuts and 2 other lands an deal 72 Damage!
Lightning Bolt Serves as your mainboard removal spell. It also serves to deal 3 damage to those pesky opponents who didn't hurt themselves with their own lands.
Primeval Titan is an alternate win condition should something happen to Scapeshift, or against aggressive decks like burn. You can play it on six lands, and so long as none of those lands happen to be basic forests, Primeval Titan can grab Valakut and a mountain to deal three damage when it enters the battlefield. So, against an aggressive deck, like burn Primeval Titan can remove an opponent's creature and block another creature, eg. Goblin Guide, and Goblin Guide.
Summoner's Pact tutors Primeval Titan, or Dryad or Foundation Breaker from your deck to your hand for 0 mana the turn it's played. The catch is that you have to play 4 mana (two of which being green, which is important) the next turn.
Foundation Breaker is a necessary inclusion in case your opponent plays mainboard Blood Moon. Despite being primarily red, this deck is weak against Blood Moon. We need Valakut, the Molten Pinnacle to win, so having it lose it's abilities does not bode well.
The sideboard is just a suggestion, but all the cards I put in the side-board are cards I liked when I played Titanshift. Damping Sphere hurts a surprisingly high number of decks, like Amulet Titan, Storm, Tron, Ad Nauseum, some Belcher lines. Obstinate Baloth is good against aggressive decks and decks that make you discard (like those that play Kroxa, or burning inquiry) Force of Vigor and Outland Liberator are good against Blood Moon and artifact decks. Anger of the Gods is good against Dredge and aggressive strategies.
When you make your own side-board, my suggestion is to use many copies (2-4) of cards that have broad effects that can hit multiple decks. Titan Shift doesn't draw many cards as their games are generally decided by turn 4, so you should want to include around 7-9 cards per matchup so that you see a sideboard card. My other suggestion is to find room for Endurance, or some other graveyard hate. Currently, the only graveyard hate is Anger of the Gods, and that might not be enough in this meta.
Overall, the deck is fairly easy to play. You ramp to 7 and cast Scapeshift or Primeval Titan. WHen you're more comfortable with the deck, you can begin adding splash colours, like Blue for Expressive Iteration, or White for Eladamri's Call and Prismatic Ending. A splash colour might be needed to gain a competitive edge in larger tournaments, but for weekly casual events, standard Scapeshift should steal a surprising number of games.
