Many rituals begin with this simple practice, especially Pagan circles. It comes from the Taoist tradition originally, I think. There are several different versions of it.
Its purpose is to allow you to feel connected to the Earth (grounded), not floating away into fantasy-world, not obsessing about the past or the future, but being present in the now. The centring part of the practice allows you to feel connected to the cosmos and the four sacred directions, which are associated with the elements.
Begin by focusing on your breathing. Don't breathe in any special way, just notice how your breath comes in and out of your nostrils, and how your belly rises and falls.
As you breathe in and out, feel your feet planted firmly on the ground. Relax your hips and your knees and imagine a thread extending from the top of your head to the centre of the sky (this helps to align your spine with the axis of the Earth).
Imagine that your feet are tree roots, and extend your roots deep into the earth. Your roots push down into the earth, through the rich soil, finding their way among rocks, and down deep into the molten core of the Earth. As you breathe out, extend your roots; as you breathe in, draw up energy from deep within the Earth.
As the energy makes its way into your body, draw it up through your legs and feel it gathering and pooling in your solar plexus. Note the colour of the energy.
Now extend a tendril of energy up your spine. Imagine that your spine is the trunk of a tree, and extend your aura at the top of your head, growing branches. Extend your branches up into the sky, beyond the atmosphere, and reach for the energy of the starlight. As you breathe out, extend your branches; as you breathe in, draw the energy down from above. Feel it gathering and pooling in your solar plexus, mingling with the energy from below.
Now draw energy from both above and below at the same time, and let the energies mingle in your solar plexus. As you breathe in, draw in the energy from above and below; as you breathe out, feel it spiralling and swirling.
Now allow the energy to fill your whole body, extending out to your feet, your fingertips, the top of your head. Feel how you are aligned with the cosmic axis.
Now acknowledge the four directions: North for Earth, representing the body, sensation, physicality, and structure; East for Air, representing intellect, thought, inspiration and breath; South for Fire, representing passion, intuition, and spirit; and West for Water, representing emotion, the Moon, dreams, and the blood that flows in your veins.