Well, if you want to just include the same "cast elemental spells" format we already have, its fairly easy to adapt. The current spells follow the current format -
Monk level 3: Cast cantrips and up to spell level 2 spells. Utility cantrips are free, damage cantrips cost 1 ki, level 1 spells are 2 ki,
Monk level 6: Access to level 2 for 3 ki.
Monk level 11: Cast level 3 spells for 4 ki and level 4 spells for 5 ki.
Monk level 17: Yay for level 5 spells! Guess how much ki?
You may increase effective spell level by 1 for each additional ki.
Its not perfect, there's some variation, but that's the general progression. Just let them pick elementally appropriate spells, then assign the level.
One thing I think that should be part of the subclass, though? There are a group of four elemental lvl 6 spells - Investiture of Wind/Flame/Ice/Stone. I think that those four spells are absolutely something the Elemental Monk should have access to, even if it breaks from the current structure.