Practice breathing in through your nose.
You want to aim to use your diaphragm, so keep shoulders and chest relaxed and drive the breath from your lower chest/stomach.
Lay on back place one hand on your chest and one on your upper stomach.
As you breathe in you should feel your stomach hand rise first then your chest hand.
As you breath out you want to let your chest hand drop first then your stomach.
Try and elongate your breath out.