Unfold a large cardboard box. If you just want a quick thrill, you could just fold up the front and sides, hold on and slide downhill. You can, however, be far more creative with a little extra effort and imagination.
Make the bottom of your sled large enough to fit your body or fit as many people you want to hitch a ride with you. You can make sides and even a top by folding and duct taping pieces of cardboard together. It can be any design that you desire - a tank, jet or possibly a dragon.
Coat the bottom of your sled with wax if you desire. This helps waterproof the sled and also helps it slide a little faster.
Paint the sides and top according to your theme. For example, if you want your sled to look like an army tank, you might want to paint it olive green and use black paint to draw wheels and chains on the side. This is purely decorative and will not affect the functionality of your bobsled.
Have fun. There are no set rules to the design of a cardboard bobsled. You are only limited by your imagination.