It's important to choose the right adhesive for mosaic work. For balls, various styrofoam figures, glass lanterns, etc. choose glues with good adhesion, for example Silicone Glues, so the pieces will not slide down. For flat bases you can choose more fluid adhesive, for example Collall, All-Purpose glue. StickyStick is helpful tool for setting pieces in the right place. You can spread glue easily with syringe or spatula.