After doing the preliminary steps to set up the adrenaline dodge move, as described in the main walkthrough—i.e., shooting the T. rex until it becomes enraged, maintaining a target lock on the enemy, making sure Lara has room to maneuver in the direction you plan to roll, and preferably setting all this up so Lara is standing in front of one of the spiked wooden barriers when the T. rex charges at her—here's how to do the actual dodge maneuver:
You can also see the whole sequence in action in this video tutorial.
NOTE: I had to use screenshots from several different battles in order to show everything clearly. So you may notice that the ammo counts, health status and positioning aren't sequential from one shot to the next. Lara only needs to fire once while performing the dodge, as shown in the third screenshot.