First of all...SoftBums are so easy to use. Like any new item, there is a small learning curve with them, but don't worry, in about two diaper changes, you will have it under your belt.
There are a few important steps to prevent leaks.
1. Tuck in the insert. After putting the diaper on baby, check the edges around the legs to make sure the insert is tucked inside the cover.
Here you can see the insert peeking out behind the leg. This will cause wicking from the insert to baby's clothes.
The insert is easily tucked back into the cover and wicking is no more!
2. Tighten legs...but not too much. I'd say that this is the hardest part to get right the first couple times. My rule of thumb is if I can see into the cover its too loose. I have found the easiest way to get the legs perfect is to put the diaper on baby, pull the elastic out of the opening, and while the toggle is in the cover, grab it and either squeeze and pull on the leg to loosen, or pull on the elastic to tighten. Doing it this way allows you to adjust it while on baby and not having to pull the toggles out of the diaper each time to adjust.