Wrap the glass bottles securely inside a towel and place the towel inside a heavy-duty trash bag. Remove air from bag and close tightly. Place the bag on concrete or another hard surface.
Put on gloves and safety goggles.
Hammer the bottles several times until your glass size is the desired consistency. Be careful not to overdo it or you will create glass particles and dust instead of pieces more suitable for mulch.
Open the bag and remove the broken glass from the towel. Place the glass pieces into a barrel or other container that can be sealed. Add a few handfuls of sand and seal the container tightly.
Roll or shake the container several times, continually repeating this motion until the edges of the glass pieces have softened and smoothed enough for use.