This also can be done any way you wish. I am just going to give my method here (maybe a few others later).
Sit down (or kneel, stand, lay down) and take three deep breaths. Try to clear your mind of the days worries, instead think on the ritual/spell you are about to perform. When you are ready close your eyes. Place your palms (sitting, laying), feet (standing), or knees (kneeling) on the floor and begin to feel them getting heavy. They are sinking into the earth, becoming one with the Mother. Imagine small roots sprouting from your palms (knees/feet) and beginning to grow. They grow longer and thicker and grow offshoots of their own. Feel the sensation of your roots moving through the dirt. Feel the coolness of the ground and wonder at the life that is present there. Let your roots grow until they find an underground water source. Feel the water being drawn up into your roots and into your body. See it as energy, purifying, relaxing and nourishing. Continue to imagine this feeling for a bit. When you are ready begin to retract your roots. Feel them shrink and pull back through the soil. Feel them disappear into your palms (feet/knees)as if they'd never been.
Open your eyes and take a deep breath. Take another breath to the count of one, and out to the count of one. Then in for two, out for two, In for three, out for three. Continue like this until you reach six, then cout backwards. In for six, out for six. In for five, out for five. In for four, out for four. Until you have reached one.
You are now grounded and centered, and are ready to begin your ritual. This same technique applies when grounding and centering at the end of the ritual