If possible, breathe in and out through your nose. Breathing through your nose slows the flow of the breath, allowing for a longer window of time for maximum oxygen absorption. If that’s difficult or uncomfortable because of health issues, you can breathe in and out through your mouth. This will not negatively impact your meditation experience.
With mindfulness meditation, the goal is to bring awareness to the present moment by focusing on the body's natural breathing pattern. If you find it difficult to notice the sensations of breathing, you can try putting one hand on your belly to observe the rise and fall of your abdomen. Some sessions may ask you to count your breath, take slow deep breaths, or notice the pause between your inhale and exhale. All of these techniques are tools to help you maintain focus on the breath, like training wheels when you’re learning to ride a bike. For different people, some techniques are more helpful than others, so experiment and see what works best for you. No matter the technique, the goal is the same: to center you in the present moment with relaxed and focused attention.