You have to download the mod (for warband 1.134) from the taleworlds forums; http://forums.taleworlds.com/index.php/board,202.0.html

For these shield battles (which didn't work out for me today because my install is giving me problems) you'll need teamspeak 3 as well. Then you just need to get online at the right time and play. They've got guides in the link above on how to fight in a shieldwall.

CR