How do you season/prepare a wood plank for Plank Grilling?
I don't season the wood at all. I buy the planks from the local grocery store or hardware store and soak in water for 8 hours. The planks often say 1 hour is enough, but I think this is just marketing, it never seems to be enough. Weighting the plank down to keep it fully submerged will help the process. 

When you're ready to cook, put the plank on a hot grill and leave it for 3-5 minutes. Once the plank starts smoking, then it's ready for your food. Don't leave it too long or it will catch fire and not smoke well as the bottom gets charred. 

You need to cook this with the lid down in order to keep the smoke contained around the food and also to keep the plank smoking rather than just catching fire. You might need to play with your airflow on a charcoal grill to get maximum smoke. 

Depending on what you cook, you can sometimes use the plank a second time. Although this doesn't work with Salmon, as the skin often gets left on the plate.