There is one kitchen utensil (?) that is essential in making this: you must have a large glass or ceramic bowl (I recommend this Pyrex 4 quart bowl). The key is to use a bowl that conducts heat. You will have a lot of trouble if you use a plastic bowl; take it from my difficult experience in grad school. A bowl that retains the heat will allow you to stir everything and get them nice and coated before the bark starts to solidify again.
Please forgive my fail of purchasing normal peanut M&Ms instead of holiday ones. I do not know where my head was... For a Christmas effect, just use green and red M&Ms.
Also, please forgive my lack of "amounts" in the directions. This is really a pour, mix and observe type of a recipe. This year I used two 24-oz packages of white almond bark, a full 8-oz box of cheerios, almost a full 12.8-oz box of rice chex, almost a full 15-oz bag of pretzels and two 12.6-oz bags of peanut M&Ms. It made lots. I filled up the 10 bags above with about 3 cups each plus a larger tin and two quart sized ziploc bags (not including everything I stuffed in my mouth while I was making it ;)).
White Almond Bark Chex Mix
White Almond Bark
Working with half of the brick of almond bark at a time (if you have a really big bowl you could probably use it all), heat in microwave for 90 seconds in a glass bowl. Remove and stir. If it is not completely melted, microwave for another 15 seconds. Stir and repeat if necessary.
Add 1-2 cups of chex, 1 cup cheerios, 1-2 cups pretzels and 1 cup M&Ms. Gently mix using a large spoon trying not to crush the cereal or pretzels. Add additional chex, cheerios, pretzels and M&Ms if you have still have melted bark that has not coated everything.
Spread out on wax paper and allow to cool.
Break into pieces and devour. Or package into pretty bags or tins to hand out. Store in airtight containers at room temperature.