Basically a line locker is a knot replacer for most situations that you would use a clove hitch in.
On the tensioning side where you wish to attach your carabiner that is the closest to the center of the line is where you would install your line locker. Typically, this is between 3 and 5 feet from the tree. There are normally only two line lockers used in a setup that incorporates them which allows your mainline to be one long span.
All that said, for a couple bucks more you can actually get a tool designed to do what you are looking for that is strength rated and has no risk or damaging your webbing:
Use a Climbing Harness Buckle (clip your carabiner into the loop):
http://www.slacklineexpress.com/catalog/product_info.php?cPath=22 - This method is proven for 4,000 lbs+
Or Use a camming buckle:
http://www.slacklineexpress.com/catalog/product_info.php?cPath=22&products_id=69