The first thing you need to do is look for a factory made contraption for your vehicle, or any other generic one you can at your local store. These come in various forms. Here are two of them:
The first is a box with a a rubber component that you place in the window, and that stays secure with friction. This is one of the easiest to install and is quite inexpensive. However, there are several problems with this. They get in your way so you should really only install them in the back seats and the passenger side. Another thing is they bounce against your car doors and rattle. They also can not hold heavy or large drinks. And the worst part about them is they fall apart easy and need to be replaced do to the fact that you will be bumping into them as you get in and out of your car. I really would not recommend getting one of these do to the nuisance they create.
The second is a box that goes across the median of your car and anchors itself there. This one is pretty good especially because it usually has a place for change, magazines, a purse or whatever you need to put on it. However it is a little harder to install because you do not want it to slip. You can anchor it to make sure it does not move while you drive. If your vehicle has carpeting you will want to do is get some Velcro. Attach the sticky side of the bur covered pieces to the bottom of the box. Then push it down on your carpet so that the burs will catch on the fibers of the carpet. You will not need the fuzzy pieces of the Velcro because they will not do any good here.
If your car does not have carpeting however you will need to attach it directly to the metal of the floor. The first thing you will want to do is wash the bottom where you are going to put the cup holders. Dirt can really mess with adhesives. Then when the surface is prepared you can either put the Velcro on both the floor and the box or you can attach the box with carpenters tape.
Now your soft drinks have a place to sit and relax while you cruise the roads in style. Now you can sip and drive without asking your copilot to hand you the drink.