The easiest way is to use one of the provided config examples if you have a board that fits, otherwise just use the example.cfg and work from there.
Again the easiest way is to manipulate the config file with your pc, log into the pi using WinSCP navigate to /home/pi, download the printer.cfg from there, edit it using notepad++ and after finishing reupload it to the pi overwriting the existing file.
If you got a preexisting Marlin / Repetier configuration just locate the pins in configuration.h / pins.h file for your breakout board you need.
Ill just mention a few important sections here:
Pins are configured a bit different from the common Marlin / Repetier scheme:
ar6 f.e. is pin d6
^ar6 is pin6 with pullup if available
!ar6 is active low / high ( can be used to reverse steppers f.e. if you change the dir pin )
Steps per mm:
Klipper uses the distance per step value to set steps per mm, just calculate your steps per mm with reprap calculator like you usually do and divide 1 by them to get the step distance.
Example: 80 steps per mm has been calculated from a typical 200 steps 1.8° Stepper with a 20 tooth gt2 pulley, now we divide 1 by 80 to get a step distance of 0,0125.
Nozzle homing offsets:
If your printer has a bed that offsets from your endstop positions ( Like an Anet A8 f.e. ) configure it as following:
This offsets the print area to -20 / -8 and brings the nozzle right to the front edge of the bed at 0 / 0
Bed heat control:
The bed heater can either be PID or bang bang controlled, i found that PID works a lot better, especially on bigger build surfaces.
In my case it looks like this:
sensor_type: NTC 100K beta 3950