You'll be happy to know it's super simple!
What to do:
Set your camera to AV mode
Choose your widest aperture (the smallest number) Mine is 2.8, for example
Take your camera off auto focus. (Put it on manual focus)
Back waaaaaay up as far as you can go away from the tree
Zoom in as far as you can, filling the frame with the tree
Play with the focus (of course you're aiming for "out of focus" here)
These pictures below are obviously not materpieces - ha! :-) They're just examples. You can see how I went more and more out of focus. It just depends on the effect you want.
My ISO is insanely high here, and I wish I would have used my tripod. It's in the back of my van from a shot I took last night, and it's too cold to go get it! ;-) Brrrrrrr!
Tip: It's important to scoot as far back as you can and then zoom in (for the most bokeh). I used a 24-70mm lens, and my focal length was 54mm. (I didn't plan it that way; that's as far as I could zoom in or I wouldn't have been able to get the whole tree in the shot).
I hope I'm not forgetting anything. Let me know if you have other questions. The Tiny Tornado is tornadoing through the house, and I'm not supposed to be blogging and parenting. Off to be with my lil' gal! :-)