Use stiff card for the stencil. Make two little holders by sticking tape together in the middle and use the wings to attach it to the card. This makes it easy to lift the stencil straight up from the piece. Do not stick the stencil to the glass. Make the stencil with only enough surrounding card to keep the whole stiff, but ensure you can pick it up easily.
If you want to use multiple stencils on the same piece you need to ensure the stencils are all of the same size to ensure you do not mark the already laid down powder or frit. You also need to make some kind of registration mark on each stencil. Registration marks are used to align subsequent stencils in the same orientation as the first. You can use notches in the stencils and always orient them to 12 o’clock or toward some other indicator. You can also use the notch in combination with a small ink mark on the glass for accurate registration.