Ever wanted to control several stepper motors precisely with just one microcontroller? Use PWM! Instead of bit-banging and writing your own delay functions to create square waves, you can use the builtin timers and pin-change interrupts available on most hobbyist microcontrollers.
First solder headers to your DRV8825 board and Teensy, then put them on a breadboard. (It's actually easiest to solder these headers while the boards are alreadyon a breadboard, but work in whatever order you'd like!)
We will connect the stepper driver much like the diagram from Pololu's product page (also included in the image below).
The teensy will have one additional connection: pin 9 will be tied to pin 3. This will allow the teensy to count pulses via a pin-change interrupt.
Also make sure to set the DRV8825's current limiting resistor to an appropriate value, as described on their product page.