Short No Leash Jungler Guide
Leashes help a lot, but aren't necessary.
From spectating, I know leashes will always be there at higher elos, but I'm B5, so I've learned how to solo even when I'm not using
and make clear times on par with challenger/diamond solo queue (only solo queue though, not even remotely comparable to clear time of a team of 5).
This is accomplished by clearing the smaller camps before recalling, then getting the red and blue before ganking, from there, you're at normal.
My strategy - start at wolf camp, don't use smite. You're level 2 Smite the gromp and kill it. Wolves and gromp down. Run to red jungle and smite the red but don't engage it (do this for the health), kill the stone thingies then the foxes.
When you recall, you should be about even with the enemy jungler even if they had a leash. From this point on, you jungle as usual.