Make sure to soak the planks well - you may need to place something heavy on top to hold them under water. The cedar plank should be smooth on one side (to place salmon on) and rough on the other (to place towards flame) for best results. If the edges of plank start to catch fire, mist with water.
Soak cedar plank in water overnight (or at least two hours), then drain.
Rinse salmon under cold water and pat dry with paper towels.
Season salmon as desired.
Set grill to medium-high heat and place cedar plank on grate, rough side down, until there is a smell of smoke, about 3 to 4 minutes.
Add salmon to smooth side of the plank.
Cover grill and cook until salmon is opaque and flakes easily with a fork, about 30 minutes or until internal temperatures reads 135°F on an instant read thermometer.