The best account of how to flatten bamboo comes from the late Dean Torges DVD Hunting the Bamboo Backed Bow. He used a jointer to rough it flat, then continued with a sharp block plane. There are a lot of subtle details that will help you avoid mistakes, and safety issues, but it would take too long to tell you, so I recommend buying a copy of the dvd.
As the saying goes "There is more than one way to skin a cat". A budget method would be to buy or borrow an electric handplane with TCT blades and rough it flat with that. TCT blades are very important as bamboo has a lot of abrasive silica that will quickly blunt your blades. You could use files or rasps, or spokeshave, or block plane, to bring it to its final shape and size. You also need to make a template for your backing or you will end up with uneven edges, too much thickness of the boo.
Like I said you could make a bow without seeing the dvd, but you are likely to waste more time and money with mistakes.
