Grounding and centering is whatever brings you back to the present, and back into your body.
Avoid going for painful techniques, and self harm.
My quickest is to look around at different shapes and colours in my environment. Perhaps counting as many of each colour from the rainbow, or looking for different shapes in different orientations.
The more of your senses you can engage, the better.
If you have them available, nice tastes smells, like aromatherapy oils, and nice bodily sensations, like a heavy soft blanket, are good.
For returning to your body, three slow deep breaths, and then concentrating on your breathing, start at the top of your head and work all the way around each part of your body, noticing how it feels and relaxing any tension you find there. It's called a body scan, and you should be able to find it as a guided meditation on YouTube, Zindal Segal does a good one.
A good way to practice rapid grounding is to ground during an emotion tweeking part of a film,especially in the cinema. It's good to be able to practice keeping a part of you rooted in the present, so if you are experiencing regressions, you can recognise that those feelings belong to then, and you are no longer there, you are in the here and now, safe.
Pete Walker's cptsd site has some good grounding tips
Also, ignore the word depression in the title, " the mindful way through depression " is very good, you'll get the full audio book off YouTube.