using your ultimate:

try to use your ult on the main damage dealers.
common mistakes:
-diving the enemy team just to try to ult their adc's. only try to ult someone when you know you will be able to bring him/her to your team and will survive. a dead skarner is a useless skarner.
-only trying to ult the adc's. when the enemy braum is superfed it can be better to ult him.
( extreme example)

how i like to start out a match:
upgrade your Q
1. let your teammates hit gromp once or twice then smite the gromp to gain the poisonous armor boost and finish him of.
upgrade your W
2.since you have the smiteboost from gromp it is now very easy.
upgrade your Q again
3. now go over to the wolves and slay the as fast as possible
4.now instead of going to the birdcamp immediatly go to your red buff , the reason you should not go to the hawk camp is that they will rek you at low levels so just this one time skip it. when you arrive at your red you should have your smite up again. fight the red until you are getting a bit low on health then smite him and continue fighting.
upgrade your W
5. if possible (should be when you used your health pots correctly) kill the golems now.
now choose if you want to go back and keep jungling until you get your ultimate or go back and gank a lane.

in the early game skarner is quite underpowered. in this fase ganking is extremele risky but rewarding. only gank lanes when you know it is safe to. for example: the enemy jungler is a bot and the midlaner clearly visible , you can gank top.

skarner is getting stronger now. in this fase when people are still in their lanes it is the easiest to carry your team. try to gank alot now (also helps for getting your devourer up). if you play out this fase correctly you have yourself an easy win.

are you having one of these games that even adds up to 50-60 minutes? don't worry as long as you keep playing smart you will only get stronger and stronger thanks to your devourer. theorethically if a match goes on for 2 hours you can instakill people resulting in an easy win.
my advice for this fase?: make sure to play smart and do not try to ult whoever you can grab since ulting that full armor and magic resist thresh won't help your team out or could even mean you lost the teamfight