Requirements: Combat 2 or Stealth 2
You can Mark a target you already have Aim on by spending a Complex Action.
For ranged attacks against a Marked target, add your Finesse or Weapon//2, whichever is lower.
As long as the target stays in sight and you are not distracted, you do not lose your Mark. However, you still need to Aim.
If your Combat or Stealth is at least 6, Marking a target is a Quick Action.