http://www.hipstersofthecoast.com/2017/03/42-land-seismic-swans-won-state-championship/

https://www.reddit.com/r/magicTCG/comments/5zu485/im_the_guy_who_won_idahos_tcg_player_states_with/

How/what do you mulligan?
When I look at my hand, I look for two or all of the following: 1. A Seismic variant 2. A card draw engine (Swans, Day's Undoing, or Treasure Hunt) 3. Temples If I only have one, it's a mulligan to six or five.

What's your sideboard plan?
Laboratory Maniac - Bring in against any deck that will side in Leyline.
Mana Leak - Bring in against Control Matchups
Molten Vortex #3 - Bring in against Midrange and Burn. The extra copy is used to get more of a controlling hand on the game.
Lightning Bolt #2 - Against Midrange, Affinity, Merfolk, Elves, and decks prominently featuring 5 toughness creatures.
Anger of the Gods #2-3 - Against Midrange, Affinity, Merfolk, Elves, decks prominently featuring 5 toughness creatures, and tokens.
Rending Volley - Against anything white or blue. Worst case scenario it's a draw 4.
Sphinx's Tutelage - Bring in against Lifegain, Turbofogs, or other decks that you can't reasonably win through damage.
Spell Pierce - Bring in against control, combo, and anything featuring IoK or Thoughtseize.
Tormod's Crypt - Bring in as graveyard hate, obviously. Also works surprisingly well against the nonbudget swans.
Smash to Smithereens - You see a lot of artifacts, you bring it in, capiche?

how did you decide on how many of each basic lands to play? 20 mountains and only 7 islands seems risky but it's hard to argue with results.
a lot of testing and tweaking. I only need/want to play 1 island per game, and 1 dual land, which usually happens.

I'm still thinking on whether or not I want Leyline of Sanctity in the sideboard. If I did, I would probably need a few Inspiring Vantages and/or Battlefield Forges in the mainboard, but I do feel like it could be super useful.

Boseiju is useless for the deck imo. producing exclusively colorless mana makes any land really risky, and Swans are usually how the deck wins anyways.

Since I need to have triple red on T3, lands that don't produce red or blue mana are not in my best interests. Source: Guy who contemplated buying a playset of Reliquary Tower for a solid 3 weeks.