Flax stem can be obtained by harvesting flax grown on fertile soil. You can gather flax seeds using the "Look for -> Wild plants" ability, that you can plant at level 30 of farming. You can plow the ground and sow the seeds you found to get a bigger quantity of Flax stems. Then to keep moving forward with the process, you need to create a Tanning tub and place your Flax stems inside and let it boil. This will give you Flax fibers. You can add 10 Flax stems at a time and receive 20 Flax fibers after an hour.