Go outside, sit down somewhere, and focus on the external world and not your thoughts. You could even do the whole “breath in, breath out, and empty your thoughts” meditation schtick — if you feel it’s necessary.
If you haven’t been able to focus in a long time, 15 minutes will be hectic. You’ll know you’re being mindful when it feels like you’ve melted into the environment — and are simply an antennae to the stimulus.
Also good for disassociative traumas — the kind that give you the “1,000 yard stare.”