Divide the batter equally between the 12 muffin cups. The batter WILL mound slightly higher than the paper cups, that’s perfectly normal — just try and shape the tops into nice rounded domes. Use a large spring-loaded cookie scoop
if you have one: it'll make your job much easier! Also, you could divide the batter into 18 to 24 smaller muffins instead, if you preferred smaller muffins. Of course, you'd need to adjust cooking time accordingly.