Wait until there is an opportunity to sit down and calmly talk to your mother. (If you are anything like me, doing it while you are mad and yelling will just make your mom defensive). This is basically what I said to my mom.
"I know that you are only trying to help and give me advice on how you think I should raise my son, but really all you are doing is making me feel like an idiot and a bad parent. It makes me think that you think that I'm not doing things properly and that in turns make me feel horrible. I believe that I know what's best for my child, since things are a lot different now then when I was little. I do appreciate your concern, but I'm the parent, and I will do as I see fit. When you contradict me it only makes me angry with you and not want to be around you. You are now the grandparent and have every right to love and spoil your grandson, but I will be the one to raise him."
After I had this conversation my mom realized that I was right and has since been trying really hard not to give unwanted advice. It's funny, because now that she's not giving advice all the time, I find that I'm actually asking for her advice more, but at least now it's advice that I want. I think that a lot of times are moms just don't get it, but hopefully by sitting down and being frank with them will help open their eyes. I really hope that this helps. I'm so glad I did it, it's made a huge difference in my mom and mine relationship. Best of luck to you! I hope it all works out!