I'd suggest practicing up on lower level groups until you get the hang of it. I used the dwarves from the pit in South Barrens to really get the hang of it. Or you can do the Cave Yeti's west of Tarren Mill. I didn't see the topic that explained this, but here's a brief list of instructions:
1. Mana shield
2. Gather x amount of mobs into a tight group, keeping mana shield up.
3. Frost Nova
4. Blink to get distance
5. Cast blizzard, placing the back edge of blizzard where the group is.
6. Place another blizzard right where group is going to be
7. Start again at number 3 until everyone is dead.
As far as talents go, max out Improved Blizzard. Avoid the talent that has a chance to freeze things in place, that will ***** up your grouping.
Edit: oops you got in before I did.
Edit2: Found the topic I used as my reference. Here:
http://wow.allakhazam.com/db/class.html?wclass=7;mid=113592100345022069;num=22;page=1 Edited, Aug 4th 2006 at 7:37am EDT by jquick