Sole juggles are far easier at first than sole stall. Also, you would like of look silly being able to toss the ball up to sole stall, but not sole juggle at all.
Practicing sole juggles you should just keep at it. Make sure you are using small touches with the ball of your foot just pushing the ball up. The reason soles are so hard at first is it's not something you naturally practice. You have no prior skill in this area. Once you can do around 100 sole juggles you can begin to try sole stall.
For sole stall, you should start out with both feet in the air as it makes balance and flexibility easier. Stall the ball on the arch of the foot. You really just have to stick at it. Once you got sole stall, you will be able to do it for a very long time.