If you want to be cheap and level it as fast as possible it is extremely easy. Just save before taking an item, if successful save again and take something else. If you fail reload. This is tedious at low levels especially on consoles.
Since I was the leader of the guild I just tried to pickpocket anyone, if I am caught I pay the minimal 12 gold bounty (I'm with the guild), then go back to stealing everything. I had the
Spoiler Guild Master's Boots +35% pickpocket
and I also had a few potions to increase the pickpocket chances.
Once I got to a certain level most items were 90%. I just spammed the take button until I took everything. If I got caught I paid 12 Gold and repeated.
Both these ways were cheap and boring, but it's very easy to get to 100. I got from 30-100 in around an hour if not less. I had tonnes of potions that made it easier to level at first though.
I only tested this pickpocket out at first. I reloaded back to my original pickpocket level as I found it weird to grind to level my skill. I prefer to do it over time.