You simply collect and focus energy with the wand, and then transmit that energy into the spell. I do this by in the beginning of the spell, I will channel the energy into the wand. At the end of the spell, I hold it in the air, or will use the gemstone at the end of the wand to extinguish to flames and let the wand soak up the smoke.
It's really any preference, just as long as the energy is channeled in the beginning, and it will automatically flow through at the end on its own if you don't guide it. You could simply hold it through performing the whole spell.