How to inflate YoYo Fun Mylar Balloon
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 it has reached its proper shape. Be careful not to over-inflate your mylar balloon or it will pop, you have to leave few wrinkles as the photo bellow.
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.