u're going to start dark seal only when you want to play agressive,usually against easy matchups (check matchups section).
Ekko's trades are based in his passive , 3 hit to wintrade XD. he deals extra damage plus gets movespeed.

Roam , participate in fights and help your jungler with objectives,all dragons are welcome in our ekko , 10% ult cdr,2.5% sustain,extra tankiness or extra damage . Last one is really bad XD.
ekko w is the key to teamfighting , it can stun multiple targets , use your R wisely not only can it gives you another life as it deals massive damage. ekko's roaming is powerfull , you can set up dives easily.

This depends a lot , when you're playing the tanky build , you're a decent carry u tank a lot u damage a lot when they think you're dying (boom) R ^^., when full ap , you're squishy and does not want to get hit by a stun silence or whatever that does not let use your R . use your R wisely either to reposition yourself,kill and enemy or deal massive damage to multiple targets.

DARK HARVEST.
Usual rune , for late game carry , i always use this when playing the tanky carry for late game extra damage.if you're thinking in the mid/late game use this.
ELECTROCUTE.
Use this agains easy matchups(check matchup section)and when you want to play agressively ,remember to have it up when going for all-in.
ULTIMATE HUNTER OR RAVENOUS HUNTER.
i use ravenous hunter in almost all champions for extra sustain but this is useable , late game with the 10%cdr dragon , your R cdr is crazy .
SECONDARY RUNES:
LEGEND-TENACITY.
Use this against teams with CC.
PRESENCE OF MIND.
for extra mana sustain against easy matchups , usefull against orianna and lux.
TRIUMPH.
good rune for 2x1 outplays , extra sustain and gold income.
NULLIFYING ORB.
usefull against most ap based midlaners , especially diana and sylas matchup.
NIMBUS CLOACK.
this rune is broken but i didn't find it very usefull.
MANAFLOW BAND.
extra mana sustain , not really needed.
GATHERING STORM.
extra late game ap . you want to go this rune unless you're playing jungle.

For the optional stats use CDR/Adaptive damage/last one is optional based in matchups.

Jungle ekko is based in his excelent gank and dive setup . his w covers a decent area , and when you're ahead is hard to stop you. level 6 dives are even easier.
gromp is kind a waste of time , unless u have nothing else to do.
when red buff is near raptor camp ,Q it and the camp to get full Sustain.

i just started top ekko , it does is kinda troll against some matchups like darius . the more i play i add to this section .