While I definetly don't think she's better there than mid in the Leona Nautilus support meta, I can provide some insight into runes and items.

For runes you have maybe 3 main choices: Electrocute, Dark Harvest or Glacial Augment. You want to generally pick the first rune but yuo can go for Dark Harvest if you believe that botlane will become a fiesta (as in tons of kills, so you can stack it faster - or maybe in non tank matchups). Glacial Augment can also be picked if you want to hit easier your W but you need to follow the GLP Twin Shadows path.

As for items you should start with the Support AP Item and then move towards Luden's / GLP based on your choice. Then you follow a normal mid build while also buying and placing vision wards (and normal wards as well). You need to get the red trinket somewhere after completing the Support Quest so you have those wards. Generally Support item - Ludens - Sorc - Zhonya/Orb->Morellonomicon -> Rabbadons/Void Staff could be a decent build. With Glacial it wuold be supp item -> GLP -> Sorc -> Twin Shadows and then one of the above items perhaps Zhonya.

For runes you have example in the rune section.