How do I steep mint for mint ice cream?
You want to make a "mint infusion". Googling this will give you many recipes.

Basically you brew the mint like herbal tea:
Remove stems,
Bruise the leaves a little,
Add the leaves to a cup of very hot water,
Let steep for a few minutes,
strain and throw away the mushy leaves and use the liquid for your flavoring.

The liquid will be brown as you are destroying the chlorophyll that makes the leaves green. In most applications I add food coloring to fix this. As you are making chocolate mint ice cream this should not be a problem.

Save some of your mint leaves to use as a garnish.

I love chocolate mint and I am very excited for you having a bumper crop of it.