Separating class code into a header and cpp file

I am confused on how to separate implementation and declarations code of a simple class into a new header and cpp file. For example, how would I separate the code for the following class?

class A2DD
{
  private:
  int gx;
  int gy;

  public:
  A2DD(int x,int y)
  {
    gx = x;
    gy = y;
  }

  int getSum()
  {
    return gx + gy;
  }
};