I retire my trusty old espresso maker during the summer months and live on cold brew coffee. Years of trial and error have resulted in my perfect method (perfect for me, I mean. You may need to experiment to find the perfect method for you.)
Your coffee needs to sit for at least 12 hours so you need to start it the day before you want to drink it. I was a little intimidated by the process at first but then I realized that I could make a week's worth of coffee in one go so it was actually saving me time.
A 8+-cup container
125g coffee (I use already ground coffee but if you're feeling fancy you can freshly grind beans) (Also, I realize the photo above shows a 250g package of coffee. I use half the package and keep the other half in the fridge until I make my next batch. This particular brand costs $5 and a batch of coffee usually lasts me about a week, meaning my entire week's coffee costs less than a single Starbucks iced latte. Warms my frugal little heart.)
Measure 8 cups of water (I use tap water because I'm super fancy but you can use bottled if you like. Just make sure it's room temperature or colder - you don't want to introduce any heat to your coffee.)
Dump your grounds on top and stir until uniformly wet. The goal isn't to have a uniformly combined mixture, it's to make sure the grounds aren't just sitting on top of the water like in the picture above.
Cover with plastic wrap and let sit at room temperature for 12 - 24 hours.
That's it! Simply strain your coffee through a cheesecloth-lined strainer and you've got a beautiful coffee concentrate that will keep in the fridge for a couple of weeks (it's never lasted longer than a week at my house but I think you could probably keep it for longer.)
To serve, I put 3 - 4 ice cubes in a tall glass, add a pump of flavoured syrup (coconut is my jam but I'm also a fan of hazelnut,) fill 3/4 full with coffee concentrate, and then top with milk.