grasp of the undying is a great standard rune option for leona as it helps in health scaling. the empowered auto attack is also a great source of damage in the laning phase.
-demolish pumps out tower damage that scales off your maximum health
-conditioning gives you built in resistances
-overgrowth is free health scaling as well

-manaflow band is great with mana sustain
-gathering storm helps you pump out more damage over time

your starting item is a tear of the goddess along with potions to help with mana sustain and later build into an important core item, winter's approach.

on your first back, bami's cinder is the ideal item as you will build it into you sunfire aegis or into your hollow radiance. but if your matchup's healing overwhelms you, bramble vest is a solid first back item as well.

your core consists of sunfire aegis if up against an ad matchup or hollow radiance if matched up against an ap matchup.
your secondary boots next. cdr boots if youre ahead. plated steelcaps if against heavy ad. merc treads if against heavy cc and/or ap.
fimbulwinter is next for the increased mana and health.
titanic hydra is your last core item as it offers damage that scales off your max health which should be high given your runes and items.

situational items are what make up the rest of your build.

ill start with your armor options.
thronmail - great antiheal, armor, and health
iceborn gauntlet - great against very mobile ad champs
frozen heart - great against matchups that rely on attack speed
randuin's omen - great against matchups that rely on critical strikes

here are the magic resistance options.
force of nature - gives you great mobility and stacking mr
kaenic rookern - great against ap matchups as the mr shield scales off max health
hullbreaker - great splitpushing item, best of both worlds
abyssal mask - great agaisnt ap matchups who also build mr

jaksho the protean - is a great last item as it provides increased resistances