The long way:
I use clarified butter on my home popped popcorn. Cut 1/2 cup of UNSALTED butter into 1 tablespoon pats. Place in a small heavy saucepan and melt slowly over lowest heat without stirring. Let simmer for 10 to 15 minutes. Remove from the heat. Strain through a fine mesh sieve (I line it with two layers of cheesecloth) into a measuring cup with a pourable lip. This will separate the solids and water from the real butter fat. It should be a clear yellow color and oily consistency. Pop the corn and then slowly pour the clarified butter over the popcorn (toss the popcorn as you pour). Salt as desired.

The short way:
Warm approximately 1/4 cup of Orville Redenbachers butter flavor oil in a microwave safe container in the microwave for 10-20 seconds. Pour over popped corn. Salt as desired.