Why is meditation so important for witches?
Centering your mind with meditation is step one before you begin any kind of intentional magic work, whether it’s writing a spell or dressing a candle or executing a ritual.
This should be a precursor every time. It isn’t mentioned at the start of most spells you’ll find, but it is an absolute requirement to stop and focus your mental energy once you’re set up and about to begin your work. Without this base level preparation of your body, mind, and surroundings you can have confused and cloudy results. How well would you expect to aim a bow wearing a blindfold? Meditation takes off that blindfold.

If we are careless with our thoughts while doing magic, there can be unintended consequences. If we use great care in what we do, we can achieve incredible precision. If we don't work to create the focus we need, how can the universe work to fulfill our desires?
To effectively work with magic the mind must be trained, and every book I’ve ever read by other magicians emphasizes that consistent meditation practice is really the only path to achieving this. George RR was right when he said minds "need books like a sword needs a whetstone", but the magician’s mind also needs meditation to stay sharp.
How to do it
Getting started is as simple as carving out time in your day for sitting, free from distractions. Open the front and back doors of your mind and let the thoughts pass on through. Behold the space left when thoughts hold no dominion. Don’t let frustration bubble up if you can’t keep a clear mind. Start with just a few minutes and work up to more time. Eyes don’t have to be closed. Many folks use a 20 minute candle, or watch the sand fall to the bottom of an “hourglass” style timer, or use a mala (prayer beads common among Buddhist meditation practices) or even a rosary (prayer beads from the Catholic faith), or a guided meditation recording. Tools like this can help bolster your practice and grow your discipline. Meditation doesn’t have to be that formal, either. If you like to “zone out” while painting a picture or doing the dishes, congratulations! You were meditating. Eastern philosophies surrounding presence of mind is where contemporary mindfulness practices originated. Do what feels right, but do carve out space for quiet reflection regularly. For those who would like to get a firmer foundation for their meditation practice should seek out The Miracle of Mindfulness by Thich Nhat Hanh.
Final Thought
When we train our minds like a muscle, concentration and focus become an act. Intrusive and unwelcome thoughts lose all their power. Success in ritual requires a mind able to give undivided attention, and with consistent practice, this will happen with less and less conscious effort over time. When attentiveness feels like muscle memory, we achieve the desired results and keep a tight focus on the work at hand, less unintended consequences, more spectacular and desired results.