"Fell Trees and create treated planks."
Find a Forestry in Othalo. This area has a lot of trees you can chop.
Forestry in Othalo
After that find a tree you want to chop and, well chop it down.
Chopping a tree
You will get some logs and wood sap, after that go to the Forestry workbench (it's very near where you cut the trees). And use it to make some timber. Also you will need to buy some Abrasive spirit from a nearby forester.
Workbench crafting window
And there you have it, you just made some timber which can be used in further crafting.