How Do I Convert My Dual Carb Setup To A Single Carb Setup?

How do I turn my dual carb bike into a single carb bike?

Do not, I repeat, do not try to turn your dual carb bike into a single carb.
I have seen several bikes become worthless scrap heaps due to people attempting to modify the dual carb Honda Shadow.
Here are the reasons why you don't want to do this -

Two carbs provide a perfect mixture to each cylinder. It's the absolute perfect means of fuel delivery for each cylinder. Honda had to go to a single carb only because of EPA regulations and as a result, they lost horsepower. Don’t believe me?... Just look at the spec sheets for a pre-2000 VLX versus a 2003 VLX. The STOCK numbers even reflect the decrease in performance AND mileage!!!!
The only way to turn a dual carb anything into a single carb without harming the motor is this: The carburetor must be equidistant from each cylinder/compression chamber. This would mean you have to put your carb right in the middle of your tank. And this then poses another problem - wind and distance of intake plenum which I'll touch upon later. Because the intakes are on opposing sides of the cylinders this is the only way to accomplish this without harming your motor.
Creating an intake can’t just be done with PVC or pipe fittings or even welded tubing. The interior of an intake manifold or plenum need to have SOME texture to the surface, otherwise fuel will pool up in certain areas. Additionally, you cant have ANY right angles in the intake. This would also cause the emulsion process of fuel to be reversed from the throat of the carb and cause large drops of fuel to collect and move into the chamber… No good. Put this to the test, go to a flow bench and pull emulsified fuel through a perfectly smooth bore intake. You will see that fuel will pool and you will see large drops being thrown into the cylinder. This now causes a symptom called "washing your cylinders" as the fuel is now much more difficult to ignite and excess fuel ends up stripping the cylinder walls of the protective layer of oil needed to protect the cylinder from the rings.
Going from a dual carb to a single reduces your performance and the aerodynamics of the bike. Why would you want to decrease your bikes ability to perform better?
Most of the intakes people attempt to build place the carburetor to the side of the bike. Now think about this. One cylinder is going to be closer to the carb than the other. Wonder why manufacturers of vehicles don’t do this? Because it’s bad mechanical engineering and harmful to the motor and will give you piss poor performance. Motors and carbs are designed to function for specific applications in conjunction with each other. Sure you can modify and improve sometimes but this isn’t a modification; it is just uneducated foolishness. I don’t care what yahoo on hardcoresickbobberbuilds.com forum built or fabbed his own intake and told you it worked great... Do you think he really invested the time in learning air/fuel dynamics for the internal combustion motor? Probably not.
When something evaporates it takes heat out of all the stuff around it. When fuel leaves the carburetor venturis, it is vaporizing (being ripped into little teeny-tiny droplets), but it is also evaporating at the same time (so much more surface area to evaporate from with zillions of little tiny fuel droplets). Because it's evaporating, it's drawing that heat needed to do so from everything around it- namely the carburetor and intake manifold. This can drop the temps low enough that when the air is cool and moist it can actually cause water vapor in the air to condense on those (chilled) parts. In really cool, wet weather (below about 45°F) or with the distance between the carb and the intake valve being too long, that condensation can actually FREEZE on those parts after it condenses, thus leaning the mixture out. When it does so inside the venturis/around the throttle plates in the throat of the carb it's called "carburetor icing" and it can choke the idle down to the point it can actually stall the engine! Ever see tiny beads of water on an intake? This is a non-optimal situation, keeping the intake close to the combustion chamber keeps the temperature warm enough to prevent the formation of ice in the carburetor throat as fuel is evaporated. Fuel evaporation can cause extreme temperature decreases in the throat of the carburetor. During particularly humid and cool weather conditions such as riding in clouds or fog, ice could form in the carburetors and prevent the engine from running properly. That is the theory. In practice, this rarely happens. There is a reason the connecting rubber between the carb and the motor is called an “insulator boot” it has insulating properties to avoid this situation.
Finally, if you want the look of a single carb bike, get a single carb bike. Don’t attempt to ignore physics and internal combustion dynamics.