Before you attempt to etch glass, you must have very clean glass. Use vinegar and water -- and coffee filters make great lint free cleaning cloths for glass.
Mac-Tac shelving paper is the stencil material to use to cover the glass. Its adhesive will keep the acid from flowing under the stencil. Layout and cutout the pattern.
Craft stores will sell an etching gel that is real practical because it does not drip. You can also use a strong oven cleaner (one that says NOT to clean the oven window with it). This is a base rather than an acid, but it will frost glass (including your oven door if you are not careful).