Step 1: Take a bicycle and build some kind of frame system onto which you attach everything else.
Step 2: Set up a gear system going from big to small to big to small to big, etc to create high gearing.
Step 3: Connect the gear system with chains.
Step 4: Put some kind of one way device such as a socket wrench or one way clutch on the back gear.
Step 5: Create massive long crank arms from which you extend from the back of the bike.
Note: there are many more steps than this, more than can be put on this post. Also note that I have tried to get this bike working correctly after 2.5 year and still have not gotten it to work correctly. If you are going to make this you probably need very high precision.