create a new folder to save contract file, for example helloworld
  enter the contract folder, then create helloworld.cpp and helloworld.hpp

  enter the contract folder, for example "cd testHelloWorld", then compile cpp:
  python ../gentool.py --type common --file helloworld.cpp
  then common.hpp will be created. next add '#include "common.hpp"' as the last "#include" to your helloWorld.cpp.
  
  enter the contract folder, for example "cd testHelloWorld", then compile cpp:
  python ../gentool.py --type wasm --file helloworld.cpp

  then helloworld.wast and helloworld.wasm will be created.

  generate abi file:
  python ../gentool.py --file helloworld.hpp

  then helloworld.abi will be created.