Step 1: Create your 'Slide
Use pliers (to bend the metal back and forth along a line until it shears) or bolt cutters, to cut the length you want.
Then, bend the brick-strap to a ninety degree angle, creating an "L" with one longer side and one shorter side.
Use any method available to you to pound the angle flat.
Step 2: Practice
Slip the long side of your slide through the door.
Work the slide towards you.
Use your other hand to pull on the door so that when the latch is out of the hole in the doorjamb , the door pulls open - Voila!
Sometimes a door latch will be stiff - wriggling, shaking, or agitating your slide might help.
Occasionally, you can use the slide to ratchet open the door by wiggling it left-to-right.
Some doors have lock-out pins, just above the latch, to make the latch immobile if the lock-out pin is pressed in. So always come up from the bottom!
Some doors have slide guards - get to it later, once you understand the basic principles.
If these coaxings don't help, practice on a different door.