The CDC has posted a good podcast on how to don an N-95 Mask (excerpt below).
N95 respirators are designed to protect you from breathing in very small particles that may contain viruses. These types of respirators fit tightly to the face so that air is inhaled through the filter material. N95 respirators should be worn only once and then thrown away in the trash.
Manufacturers supply important instructions with respirators on how they are to be put on and checked to make sure they’re properly positioned. Always follow the manufacturer’s instructions, specific to the product you’re using.