My MIL is a nice woman but she has the habit of instructing me all the time. Like, she all the time tells me to wash hands when I go to the kitchen, when we are done with our meal, she wants me to clean the table right away. I am a laid back person. I do everything at my own pace. No matter how much I try to clam myself down, at times it becomes quite annoying. How do I tell her that I have a way of living and do not interfere in that?

Be patient. What I have learnt from my experience is that we should not become agitated with small things like these in life. How does it matter if you do something 5 minutes early than what you like to do. I mean unless it is creating huge problems in your life then speak up, of course, but if doesn't matter, do it. With time she will also understand that she should not push things for no reason. As you said, she is nice, then you can sit with her and can make her politely understand.