This is easier for me since there is a bamboo forest next to me but, find a piece of bamboo and fit it to the size you want your bong.
Drill a hole in the side with a small bit and put it at an angle. Then keep on drilling with bigger bits at a downward angle and you will have your hole and this is where you put you bowl.
If you want a stem then get a glass one and put it in since bamboo stems are shit.
Tips: You may want to scrape the inside pretty well so its just the hard wood and no dust shit comes off.
Let me know if you have any questions!