Instructions
Go to your career trainer once you have gained a specialization point, which you start receiving at level 11.
Select "Buy Mastery Abilities" from the career trainer's dialog.
Choose the specific mastery ability on which you wish to spend your specialization points and confirm the purchase.
Select your mastery tree from the right panel on the career trainer's menu if you wish to spend specialization points on your existing mastery abilities. Click the up arrow next to the ability to level it in this fashion. Close the dialog box when you have finished spending your mastery abilities.