Cold brew is my favorite type of iced coffee. In the spirit of diy I usually brew my own homemade stuff. And in the spirit of open sourcing everything I thought it would be nice to document my personal "recipe".
Searching around you'll find many different ways of making cold brew and the truth is that there is no right or wrong way of doing it (well... except some very wrong ways of doing it). This kind of brewing is fairly new and different people follow different rules. Especially when it comes to the coffee-water ratio. You should experiment with different ratios and find what's more suitable to you, in terms of flavor and caffeine amount. This is just what tastes good for me.
You need coarsely ground coffee. More coarse than you'd grind for filter coffee. If you have your own grinder you should be able to achieve that easily. I avoid brewing more than 100g each time. Otherwise I'd need a really big bottle to store it when it's ready. So use your scale wisely. Depending on the kind of bottles or jars you have (and your fridge can hold) you may end up with a different amount of coffee to use. You'll have a better way of deciding this by the time you end up reading this post.
I use a jar for that. One with a cap to cover it and keep unwanted smells outside. My volume ratio is 1-4. That means 1 part of coffee and 4 parts of water. I don't measure water, I know the jar's total volume so I can do it visually. If you want to be precise (I'm not), you should know the exact volume of your jar and the fact that roasted coffee's density is around 432kg/m3.
So fill in water and stir it good. You can find alternative ways for that specific part. Some people enclose the coffee inside filters or other kind of pouches, so they can remove it easily later. I prefer to filter afterwards, avoiding any other material to be part in this process and let the coffee brew directly within water.
You can either leave it on room temperature for 12h or put in the fridge for the next 24h. I prefer the latter, slowing down the brewing process. It's important to stir the coffee another 2-3 times during these 24 hours.
I filter directly to a bottle, using a filter paper (like the ones I'd use for filter coffee machine) and a funnel to make sure I don't miss a drop. If you don't have filter papers, you can use a wet napkin that you wrung before using it, but the filtering process will be slower.
Once you filter it all, you store the bottle inside your fridge again.
Cold brew is a lot more caffeinated than your regular espresso or other forms of iced coffee. That's why you should dilute down your cold brew with water. My ratio is around 1-4 again. Cold brew is served cold (I know, right?). That means you can use a lot of ice cubes and a bit less water. Usually I don't like milk in my coffee, but cold brew is an exception. Milk bonds really well, and you can use it instead of adding water (or use both).
The result is a nice coffee drink, less acidic and full of flavors compared to other forms of iced coffee, and a long lasting taste of pride for brewing your own coffee.