This technique relies on distraction. By ‘moving’ the memory, one can break away from the constant loop of anxiety, causing meditative techniques to work well.
You can find an interactive guide to grounding below.
If you see something that they shouldn’t, taking a picture with their phone camera and then moving away from the object may help your brain realise that thing does not exist. Optionally overlay a grid or another object over the picture if you have the ability to do so.
If it is safe to do so (so, not in a dangerous neighbourhood, situation or very late at night), walk outside your house and touch any objects you can. House, lamppost, bin, it doesn’t matter. Make sure you study the objects while you touching them, and seeing what they are made out. Make sure you wash your hands afterwards!