Default configuration assumes 20 Mhz OSC and 19200 baud. This can be changed in the file "avr_cfg.h".
When the boot loader starts it will write:
www.eit.se/habl/[version]
Then it will wait 3 seconds for user cancel autoboot by pressing Esc exactly 5 times.
If Esc is pressed 5 times within 3 seconds it will start sending ready for xmodem: CCCCCC...
A binary file with an application can now be sent using xmodem protocol.
If you have a hex file you need to convert it to binary first, "hex2bin" can be used for that. It can be found at Atmel Corporation.
If any other key is pressed during the 3 seconds or esc is pressed more or less than 5 times then the application is started (autoboot).
If there is no application loaded it will probably go into cyclic reboot until an application is loaded. You can test it by loading our hello world program.
By default habl enables the watchdog timer so the applications loaded with habl needs to support that. This can be changed in avr_cfg.h.