Self Locking Webbing, Slack Line Method

Step 1: Use a Line Locker for the guiding-line side then run the webbing through a carabiner on the anchor side.

Step 2: Run the webbing through the carabiner on the guiding-line side from beneath to top then through the carabiner on the anchor side from top to beneath.

Step 3: Run the webbing through the carabiner on the guiding-line side from beneath to top once again.

Step 4: Slip the upper bend of webbing under the lower bend of webbing.

Step5: Here is a side view.
Remember to tie off the system so it cannot accidentally come undone.