How do I avoid burning the butter and marshmallow (I tried melting butter and marshmallow on the stove)?

I have done this too many times as well. What I eventually remembered to do is to allow the butter to cool slightly before adding the marshmallows. Remove the pan from the heat and let it stand a minute (or two depending how high the range was when you melted the butter). Add the marshmallows and cover for two minutes. Gently stir. If the marshmallows are taking too long to melt, or the heat has evaporated, turn the range to medium-low, and gently stir the mixture until melted, removing as necessary before it bubbles. Let it cool according to the recipe before adding the cereal. A little more work, but worth the time.