The build is meant to have you as a tank as well as a support. I have you sit on the gp 10s for a long time because overall this gives you double the passive gold gain which is helpful when you arnt farming. The build makes you very tanky and very resistant to cc which allows for more peeling and overall interuption of the enemy. You have very high early game physical resistance which later on becomes very heavy magic as well. The frozen heart can be swapped out for a warmogs if there ad is doing very badly or if hey are a heavy ap team for a crucible for the mr as well as to remove any cc's on your highest damage ally.

The build order can be moved around a bit like getting the glacial shroud before the aegis if they have a fed ad but generally this works very well for tanking while peeling and ccing for your adc.
The reason i start with only 1 ward is because you will need the regen early to be able to constantly poke. And leo always lanes 1 of 2 ways. you can't win so you sit under tower with adc while they farm. or you keep them hard pressed and against thier tower away from the farm in which then if your adc only last hits you can deny the other team cs and exp. so you would only have to really ward dragon and later place them at dragon and your own tri bush. You dont need any for lane bushes especially if your winning because Leona is very good at punishing people who try to force her out.
Please remember to still periodicly buy wards and counter ward with pinks. The obvious fact that people over look is that the sight stone was placed to help releave the cost and weight of buying wards on supports. not entirely iliminate the need.

you want q and lvl 1 for the stun and the e at lvl 2 for the initiate but after that you want to get up your w as quick as posible for the massive amounts of armor and mr that it gives. when you dive with the shield on besides the added damage it also helps clear minions so you and your adc take less damage during the fight as well as getting another sunlight proc off if your adc has a fast hitting skill like corki or miss fortune. the q doesnt increase the duration of the stun and deals less damage than e so i recomend maxing it last.

I recomend 0/16/14 masteries. I have the 14 in the utility tree so that besides having more mana the wards are better, you have faster summonder spells like exhaust and have the extra starting gold and gp10 since you are not going to be farming. the 16 in the defensive tree are so that you can still be very tanking even while building support items. if you have noticed i have the tenacity boots and masteries as well as the mastery to reduce slows that its very hard to keep you away from thier carry or from running away. The running away comes in handy because if need be you will want to give your adc time to retreat safely in the laning phase before you make your escape to ensure the adc survives.

For runes i have heavy armor and mr oriented runes. the flat armor are so that especially early on in the laneing phase you can take a beating without having to retreat while as the mr are scales because you normally dont run into much magic damage untill later which gives you the time to wait on getting the effects of them because you don't really need it early. and he gp10 quints are for extra gold which always helps as a support.