Pop your popcorn first!  I always use air-popped popcorn, and I popped 3/4 c. popcorn kernels. This will make a little more than you need, but then you'll just have a little leftover to munch on.

Measure out 16 cups of popped corn into a large bowl that will be big enough mix it all together.  If you don't have a popcorn maker, you can definitely use microwave popcorn instead. It may be a little more yellow and taste a bit different due to the butter, but it will still work.

In a large pot, melt the butter over medium heat, then add the marshmallows.  I stir pretty constantly, to make sure the marshmallows don't burn on the bottom. Once it's completely melted, pour on top of your popped corn, and stir until every piece is covered.

Add your M&M's, and gently stir them into the popcorn, making sure they are throughout the whole bowl. Spoon the popcorn mixture into a lightly buttered bundt pan, and press firmly.  (Don't use cooking spray--it will make it taste funny.)

You don't need to let it sit for very long--only about 5 to 10 minutes, then you can turn it upside down onto a cake plate.  You will probably need to take a table knife and gently go around the edges, then use your fingers to loosen it a little.  Once  it's loosened, it will come out pretty easily.