Hot garbage-Diana, Fiddlesticks, Kindred, Sejauni, and everyone else who isn't on this list.
Purple Side. Blue-Gromp-Red
After first jungle path look for opportunities to gank such as, over-extended or pushed lanes, lanes without summoner spells, low HP enemy laners or unaware enemy laners. Around this time(3 minutes) the enemy jungler is also looking to gank, look at your allies, are they over-extended or at a high risk of getting ganked, warn them and possibly setup a counter-gank(these can be very effective and heavily shift the game in your favor). If you or your ally see an enemy blow a summoner spell, ping it and remember it, can setup a gank later before it comes back up.
After a successful gank
After killing an enemy laner, you want to hard shove the lane to put minions under their tower to cost them XP, gold or cost them their teleport or lose their tower. Now if the enemy jungler doesn't come to stop you guys from pushing then he's an idiot. Same for you, you want to stop them so you can get the XP and gold so it isn't wasted. Also after killing bot lane or killing the enemy jungler, you want to get an objective, be that tower or dragon.
How to keep snowballing
You want to maintain the lead you have by taking out laners without it costing you deaths. Also constantly counter-jungle to put the enemy jungle even further behind(You need to do this, makes them completely irrelevant). Just keep getting objectives, kills and be aware of the map. If any of your allies or you are alone in a lane or in their jungle without vision of the enemy team, LEAVE!! or gain vision around the area to be safe.
If your team is smashing and just wrecking house, farm and get objectives. But right as you see a enemy champion walk to another lane with allies, head there and hang up back to see what happens, if the enemy team try to fight and think they have the upper-hand come in and beat they asses.
Smite this is a very important tool, never blow it unless you need health, stealing or confirming an objective or buff. If you can farm a camp and lose very little health, DON'T SMITE IT. Save it to use it to steal an objective or buff, or if you have blue or red smite, save it to secure a kill.
If you secure kills or an objective and have very low Health or a lot of gold, just back to base and spend that gold and reset. More items you have the stronger you are, SPEND THAT GOLD!!
Never really try to 1v1 the jungle if they are playing very good 1v1 champions such as Kha'zix, Rengar, Elise or Graves. If you are extremely ahead or have someone with you, do it.
Jungling is all about map awareness, you need to be looking around the map a lot and glancing at the minimap a shit ton. You need to know all the information you need to make the best next decision.
You want to fight when you have a number advantage, never fight outnumbered, too risky and high variance.
Also if one of your laners feed their opponent an insane amount of kills like 4-0 or 5-0 and your not that strong, then dont even try ganking that lane, just gank other lanes before the fed one helps their team.
For the fags that want to play Quinn and Talon jungle, kys.
So Talon jungle isn't good, slow and sloppy clear time and ganks are pretty bad. To do good, you need to kill the top lane early if they are a tank, after gank bot only, fuck mid lane, too short of a lane and they see you slowly jumping a wall. Counter jungle when the enemy jungler's buffs are up, camp it and wait, make sure you have vision and know where the rest of the enemy team is before trying this. Plus assassinate the adc if they are farming a side lane alone, warn your team that you aren't grouped with them so they don't engage a 4v4 that they might not win without you.
Quinn jungle also has a bad clear time, and her ganks pre-six are awful, power farm till six and you can gank so many lanes in a short amount of time it's autistic. Quinn is one of the best skirmishers, counter jungle a lot as well. Plus assassinate the adc if they are farming a side lane alone, warn your team that you aren't grouped with them so they don't engage a 4v4 that they might not win without you.