1) Count your breath. Sit or lie, comfortably, and count every inhale/exhale set. When you get to 10, start over at 1. If you lose your place, start over at 1. If you suddenly realized you stopped counting and were thinking about what you were going to eat next, start over at 1. Do this for 5 minutes at first, and work your way, a few minutes more at a time, up to 20 or 30.
2) Count the length of your breath. Sit or lie, comfortably. As you inhale, count (maybe every second, but the exact timing isn't important), and do the same as you exhale. So, many people may start with a 3-count (3-second) inhale and a 3-count (3-second) exhale. Watch that pattern for a while, and then start to lengthen the exhale, one count at a time, taking a few minutes every time you add one count to your exhale to repeat that pattern. Repeat up to three times (or whatever gets you to an exhale that is twice as long as an inhale). Any time that you feel short of breath or anxious from the breathing practice, stop counting, breath normally, and try again after a few minutes.
(The point of these is to keep a long breath and a long exhale, to reduce the activation of the sympathetic nervous system (flight-or-fight stress hormones) and increase the activation of the parasympathetic nervous system (the relaxation hormones).)