To put the tinyK20 board with the NXP Kinetis K20 into bootloader mode, well someone could check the schematics, or follow this quick guide :-). In short, the pin PTB1 has to be pulled to Ground (GND) while powering the Kinetis K20. The pin PTB1 is on the outside row as below:
GND is easy to find. The PTB1 pin is located between the 8 MHz clock and the big C near the CPU corner:
I usually use a bread board wire, put into the holes, followed by powering the board with the USB connector:
Then the blue LED should start blinking and the board enumerates on the host in bootloader mode, ready to be flashed with a new firmware.
Happy Bootloading 🙂