I recommend an 8:1 water to coffee ratio. At this ratio, I usually grind 100g of coffee and that lasts me about a week, which is all you really want to keep cold brew around for. As for the coffee itself, this is of course up to personal taste but I don't like to waste money on top notch roasts for cold brew. I also won't buy the cheapest, burnt-to-a-crisp coffe either. I like to go middle of the road with a medium roast Starbucks or something I can find on sale. I have also been known to use up the beans I didn't particularly enjoy from any of my other brewing methods. As for the grind, shoot for a little finer than a standard drip coffee grind. If you buy pre-ground coffee, that will work too. Generally, cold brew coffee is more forgiving so this isn't as crucial as brewing espresso or using a Chemex.