Clean the bottle thoroughly with bleach. After the bottle has been sterilized, rinse the bleach out with warm water. If you do not have bleach available or are working with small children, you can substitute the bleach with disinfectant soap.
Drop small pebbles into the bottle to create a 1-inch layer. Fish tank pebbles work especially well for this project. The idea is for the pebbles to work as a water drainage system in the bottle.
Spoon 1 inch of potting soil on top of the pebbles. Shake the bottle gently to let the soil settle.
Drop a few flower seeds on top of the soil. Spoon 1/2-inch of soil into the bottle to cover the seeds.
Place the bottle in a warm place out of direct sunlight. Because beer bottles are made from glass and glass can act as a magnifying glass, placing the bottle in direct sunlight could cause your flower to overheat and wilt.
Water the flower when the condensation on the inside of the bottle has evaporated. The beer bottle acts as a miniature greenhouse, so your flower will not need to be watered often.