Pop your popcorn with an air popper
Melt your white chocolate (Can be done in candy melter or microwave, stir frequently, if you over heat, and it gets crumbly, add oil or shortening to get desired consistency)
Drizzle white chocolate over popcorn, and mix in the M&M's, crushed candy canes, and pretzels while chocolate is still wet.
Drizzle more white chocolate on, stir while doing this for a nice even coating.
Sprinkle with holiday sprinkles.