The Wise Woman is great, it's my favorite sling!
It's pretty close to foolproof.
About the kangaroo carry -- that's when baby is sitting cross-legged like a little buddha in a hammock, kwim? I know that it seems very scrunched and cramped, but if the baby doesn't mind, don't worry about it! Don't forget, not long ago she was even more scrunched and cramped in your womb. My kiddo loved the kangaroo carry even tho it looked very uncomfortable to me, as an adult. But then I'm not used to hanging out in fetal position all the time. It's the same theory as swaddling, most small babies don't need or want to stretch out and flap around.
As TBone got older and did want to stretch out more, we switched to the hip carry. Now that he's even bigger and heavier, I usually carry him on my back. See, you can't do all that w/ the bjorn.
Also, you can use your sling for so many other things... It's a baby blanket, a cover to protect from rain, sun, or wind, a leash, a high chair, and a burp rag. And try nursing a baby to sleep on a cramped airplane in a bjorn?
Oh, as for feeling secure -- that's just practice. Just keep practicing, walking aroudn the house and keeping one hand on her until you feel secure. You and she both will get a sense of what works for you.