Small mylar balloons are manufactured with a valve up inside the stem at the bottom of the balloon. The valve has to be opened to inflate or deflate the balloon. The easiest way to open the valve is to insert a small straw into the hole and push it upward until the valve is open. You can inflate the balloon by blowing into the end of the straw, or use a helium tank to inflate the balloon. Mylar balloons are full when most of the wrinkles are gone--they don't expand like latex balloons and will pop if overinflated.
Find the inflation opening. The stem of the balloon will have a hole on one side. This will give you access to the one-way valve inside.
Push your straw into the stem. Put one end of the straw into the opening and push it gently upward into the balloon. The valve is usually at least a few inches deep into the balloon. You will feel resistance when you reach the valve.
Open the valve. Push gently past the resistance so the straw opens the valve. Blow gently on the straw and see if the balloon starts to inflate. If it doesn't, push the straw a little farther and try again until the balloon starts to inflate.
Inflate the balloon. Blow through the straw until the balloon is properly inflated. A mylar balloon is properly inflated when most of the wrinkles are gone and it has reached its proper shape. Commercial valves used to inflate mylar balloons use only one pound per square inch of pressure. Be careful not to overinflate your mylar balloon or it will pop.
Remove the straw. Either cap the end of the straw with your finger or thumb and pull it out, or pull the balloon off the straw while you are still applying some air pressure with your mouth. When the straw is back out of the valve, the balloon will seal itself and not deflate. There is no need to tie the stem to keep the balloon inflated.
Inflate a mylar balloon with helium. Place the opening of the valve around the nozzle on the tank regulator. Seal it firmly with your fingers and turn on the gas. The pressure from the gas will automatically open the one-way valve and inflate the balloon.
You can also deflate a mylar balloon by sticking a straw inside past the valve. You will feel air rushing out when you have pushed past the valve. Deflate the balloon by pressing it flat, or suck the air out through the straw with your mouth. This is a great way to store and recycle mylar balloons of any size.