I'll give some tips. Warning though this is really long. tl;dr at bottom.
Right now I'm playing ap attack speed leona jg (
!!!)but I think the early clear is the same for both ad and ap leona. I used to play ad tank myself in season 5 so I have some idea about that too. The only real difference is where your damage comes from. I'm assuming you like ad tank so it should be pretty similar to my early clear except for the damage source.
For ad tank runes, take attack speed quints or reds, and flat ad for the other. Leo's early clear mostly comes from her auto attacks. (Taking ap instead doesn't make a big difference early because your q is just an enhanced auto but ad is better for tank since you probably want to save your abilities for engage/peel). Armor yellows are the only ones I've tried and I don't see what other seals you could take. For glyphs, cdr, ap, or mr are good choices, but if you want to have really good early clear, take ap blues for sure. It's just that leo's ult has a short cd and as a tank you may want mr; jungling isn't just about that level 2 clear.
For masteries, I would go 12/0/18 or 0/12/18. The idea is that you want good early clear so for 12 in ferocity go attack speed, feast, natural talent, and oppressor. For 12 in cunning go savagery, secret stash, either mana or dmg below 40%; doesn't matter, and dangerous game. For 18 in resolve just make sure you take tough skin and insight, anything else is up to you.
Compared to other junglers, Leona definitely has awful clear. Although during season 5 her clear was way better (not sure why), and I could go for a level 2 gank guaranteeing first blood.
Now though, with all the changes I find myself struggling for the first clear. In fact so much changed that I'm not sure whether 
is better than 
for ad leo anymore.
After testing out a couple starts I find 3 
(not the refillable one) and 
and starting on the red buff side pretty decent. (Krug smite is a must have with your 3 quick autos). Make sure you only use 2 pots, you will probably see yourself almost dying to red buff but that's alright. allows you to heal off scuttlecrab so go to that next and after killing it you should get at least a quarter of your health back, maybe even enough for a level 2 gank if you take zenith blade second. (I take eclipse level 2 but I don't think it makes that big of a difference).
It takes 5 camps to get a level 2 jungle item (I suggest red smite) so I go to birds and wolves after that, remember you should still have one left. I find that the level 2 item really helps my clear so much that I don't even bother going refillable potion because I never go below 75% health. My clear is still pretty slow though. At this point I would say early jungling is over for you.
But by then only 5 minutes passed probably. If you think your clear still isn't good then go gank a lane and get a 
, this should help. And if you want even faster clear...
or
is for you. (If you ever want to try ap 
is exactly the same, just needs 4 autos).
tl;dr
Ad/attack speed marks and quints (one for each), armor yellows, mr/ap/cdr glyphs. 12/0/18 or 0/12/18 masteries, make sure you take the ones that help vs monsters and don't take the ones that don't include the word "champion" or increase damage to you. Start krugs then red buff with 3 (not the refillable one) and . Heal off scuttle. Try birds and wolves after. Take red smite and either or . Don't forget early ganks if you can. Should be on par with enemy jg after that unless they are really good early game 
or make cheese plays 
. Watch out for them; it will be hard to fight them if they get ahead.
