Know the hazards associated with any tools, materials, adhesives etc you are using, and take appropriate care.
You must wear safety goggles whenever you are breaking up mosaic materials. Chips can fly off at high speed over surprising distances.
A dust mask will protect you from breathing in dust from broken ceramics etc, or when mixing cement grout.
Don't bring food or drink into the area where you're working.
Make sure you've got access to clean water and plasters just in case you do cut yourself.