To center, draw the energies into your body into your abdominal region, some like the “belly” area, some prefer higher or lower. Or you can store them in one of your chakras if you know were and what they are. Most people chose to center in the “belly” region simply because that could be concidered the center of the body. Draw in the energies until you feel full. When I center I usually visualize the energy filling a balloon or beach ball, when the ball is full I stop drawing in the energy.
Grounding is just like it sounds, you ground the energy, return it to the earth. With your feet touching the ground (for the first few times it may be easier to do bare foot and outside in the grass). Visualize that you are a tree, you have roots coming from your feet going deep into the ground, the sun above you is nourishing and your branches and leaves streach up high toward it.
