1. Development environment preparation

1. install "Arduino Software IDE" for your preferred operating system
   https://www.arduino.cc -> Software->Downloads
   it is strongly recommended to use older version "1.6.8", by which we can assure correct compilation results
   note: in versions 1.7.x and 1.8.x there are known some C/C++ compilator disasters, which disallow correct source code compilation (you can obtain "... internal compiler error: in extract_insn, at ..." error message, for example); we are not able to affect this situation afraid
   note: in the case of persistent compilation problems, check the version of the currently used C/C++ compiler (GCC) - should be 4.8.1; version can be verified by entering the command
   avr-gcc --version
   if you are not sure where the file is placed (depends on how "Arduino Software IDE" was installed), you can use the search feature within the file system
   note: name collision for "LiquidCrystal" library known from previous versions is now obsolete (so there is no need to delete or rename original file/-s)

2. add (UltiMachine) RAMBo board into the list of Arduino target boards
   File->Preferences->Settings
   into text field "Additional Boards Manager URLs"
   type location
   "https://raw.githubusercontent.com/ultimachine/ArduinoAddons/master/package_ultimachine_index.json"
   or you can 'manually' modify the item
   "boardsmanager.additional.urls=....."
   at the file "preferences.txt" (this parameter allows you to write a comma-separated list of addresses)
   note: you can find location of this file on your disk by following way:
   File->Preferences->Settings ("More preferences can be edited in file ...")
   than do it
   Tools->Board->BoardsManager
   from viewed list select an item "RAMBo" (will probably be labeled as "RepRap Arduino-compatible Mother Board (RAMBo) by UltiMachine"
   note: select this item for any variant of board used in printers 'Prusa i3 MKx', that is for RAMBo-mini x.y and EINSy x.y to
   'clicking' the item will display the installation button; select choice "1.0.1" from the list(last known version as of the date of issue of this document)
   (after installation, the item is labeled as "INSTALLED" and can then be used for target board selection)

2. Source code compilation

place the source codes corresponding to your printer model obtained from the repository into the selected directory on your disk
https://github.com/prusa3d/Prusa-Firmware/
in the subdirectory "Firmware/variants/" select the configuration file (.h) corresponding to your printer model, make copy named "Configuration_prusa.h" (or make simple renaming) and copy them into "Firmware/" directory

run "Arduino IDE"; select the file "Firmware.ino" from the subdirectory "Firmware/" at the location, where you placed the source codes
File->Open
make the desired code customizations; all changes are on your own risk!

select the target board "RAMBo"
Tools->Board->RAMBo
note: it is not possible to use any of the variants "Arduino Mega …", even though it is the same MCU

run the compilation
Sketch->Verify/Compile

upload the result code into the connected printer
Sketch->Upload

or you can also save the output code to the file (in so called HEX-format) "Firmware.ino.rambo.hex":
Sketch->ExportCompiledBinary
and then upload it to the printer using the program "FirmwareUpdater"
note: this file is created in the directory "Firmware/"