The first step is to flash the SiFive E21 core on to the FPGA. To do this, connect the ARTY board over USB and run:
$ make flash-e21
It may display that many memory locations are incorrect. This didn't seem to matter when I tried this.
After that has finished, press the PROG
red button on the top left of the board. After a few seconds, one of the RGB LEDs should start pulsing colors. It also prints over the serial connection. To see that, run:
$ tockloader listen
and select the option with the larger number.
To load a new kernel on to the board, run:
$ make flash