It's fine if you try to open the bottle while reading this, so go buy several bottles of champagne to practice. You'll likely open some really expensive champagne bottles with this technique so you should practice with at least 4 bottles. First, go buy a case of really inexpensive champagne brands, and a normal-sized kitchen knife. You don't need a big kitchen knife, but it should have a thick blunt side. It's important the bottles are cooled really well.
First step is taking out the protective foil, then we have two options : removing the wire cage, or keeping it on. If you keep the wire, you have to agitate the bottle of champagne really well to make pressure in the bottle. If you take off the wire, it is fundamental to not agitate the bottle because the cork could come out when you won't be prepared. If you're doing this for the first time, choose the first alternative. It's easier to pop and to practice it. Take a piece of dry cloth and clean the bottle. Make sure it's dry.
Find the seam along the bottle. The seam is where the two halves of the bottle join. Take the kitchen knife and position it on the seam. It has to be with the dull side onwards. Position the blade at a small angle with the bottle. If the angle is too large, you'll probably fail the blow. When you're set to take the first shot, keep the bottle in the left hand and with the other hand slide the kitchen knife firmly with a quick and powerful blow until the bottle cracks. Usually the top of the bottle will crack and fly with the cork. Consider the fact that with each miss, the bottle moves and the pressure level grows, so direct the cork away at all times.
If you prefer not to take off the wire cage then you're going to need a stronger strike. Use a bigger kitchen knife. Napoleon's generals used their blades to crack one of the most expensive champagne. If you have a blade or a spade, you'll certainly draw a lot of attention at your party. But for us, the everyday people, sticking to a normal knife will do just fine.
This should be enough to get you started. Grab several champagne bottles, pick the champagne brands carefully and begin practicing. Practice makes perfect. We gave you the info, but it's up to you to open one of the most expensive champagne bottles at your party using a plain kitchen knife.