Assemble the wheels and body of the bike (even though the pic shows half!) then cut down between the front spokes and fuel tank along the line shown.
Then trim down as shown.
Remove the little skulls on the sides of the fuel tank and file down the top as shown (allows the handlebars to turn).
Trim these bits out of the back of the spokes.
And you get this.
Cut out a piece of sprue (this is the the wheel sprue) with a large cylinder attached to a bar.
Then you have this.
Trim down the end of the bar (slightly).
And round off this bit of the cylinder a little.
Then you put the bar into the hole in the fuel tank, at this angle. It can be tricky to get the angle right, so try dry fitting it a few times.
Once you've got it glued in position add some green stuff to bulk up the new hinge.
And from the front.
Make sure the handlebars are attached to the spokes and you have your hinge.
Then it can turn realistically.