Grounding, Centering, Shielding. These three processes are often lumped together in various combinations of interchangeableness, however they are different things.
Grounding is akin to being plugged into an electrical socket, one has access to all the energy they need in infinite supply, but also are plugged into an infinite sink so that they don’t get overloaded with excess energy- plugged into the ground wire as it were.
If one picks up lots of random ambient energy, and is carrying excess energy, a useful technique is visualizing a lightning rod, just a big steel pole that instantaneously draws the extra stuff down to the ground. It may be useful if one is flooded with emotions, or if one is feeling air headed and floaty, a common symptom of being ungrounded.
A guided meditation demonstrating the tree roots technique
You are next to a large tree in a snowy clearing. You sit with your back to it, and think about the roots extending down into the earth, warmed by the insulating soil and it’s own respiration. Follow the roots down with your feeling, slowly extend down, down to the soil where the roots gather moisture and nutrients. Feel connected and nourished by the earth. Drop your awareness down through the soil to the bedrock, and feel the firmness of it. A big expanse of stable rock stretching in all directions and protected by the above soil. Go back to the roots and feel the stability in the whole network. Feel the vastness of the trunk held firmly by the foundation of roots. Move your awareness up the trunk and feel the air brushing your branches. Feel the movement of the air, appreciate it’s dynamic nature, feel the aspect of sky it carries. You are connected to the earth and the sky. Open your eyes.
Physical actions that help grounding are touching things, tables, walls, the ground. Touching a tree. Hematite is a stone associated with grounding. Just remember to be in the present with all of your awareness, and feel connected. Grounding is traditionally earth energies, but one can also draw on other energies: sun, air, ocean.
I always think of the dune mantra for this one. Centering is about calling all the bits of yourself together, and instantly relocating your awareness to yourself, not looking at anything beyond yourself. It’s about being in the here and the now, with all external stimuli going around or through you without sticking. I like it for instant calming.
From personal experience I would recommend trying to center first before shielding or grounding in demanding situations. Of course your mileage may vary.
On the most basic level, it’s setting a boundary between you and not-you. Heavier shielding is more protection. There are many things one can do with shielding: cut off all emotions/stuff coming in or going out, let only certain emotions go in or out, be aware of external emotions without letting them come in, etc.
A common one to get you started is the cocoon of white light.