How do I know when my cookies are done?

I make cookies for my grandma every day. But I never do it right. Every time I cook them the bottom is black. My grandma still eats them, but its like, when you want to make the very best for the people you love. Also don't be mean, I'm only 13 okay. This is the way I cook the cookies: I preheat the oven to 350 degrees. Then I put foil on the pan thingy that I'm using then I put cooking spray on the foil. I don't measure the cookie dough. I roll the cookie dough in to balls and bake them for 15 minutes. And when their done I take them out quickly and put them on a plate and serve it. Come on I have no idea whats wrong with it?