Add 1/3 milk mixture, mix on low until just combined. I usually just pulse it for a few seconds. You do not want to overmix this or you'll end up with tough muffins.
Add 1/3 dry ingredient mixture, mix on low until combined. Then repeat! 1/3 liquid mix, combine, add 1/3 dry mix, combine, repeat. Lastly gently fold in any other flavors (see ideas below).