Complete the Special Research that unlocks Zygarde
Use the Routes feature and walk completed Routes
Tap the glowing Zygarde Cell that appears along a Route
Check Routes daily, since Zygarde Cells can appear at random
Finish the Route to have a chance to collect the Cell
Walk different Routes to increase your opportunities
Keep an eye on the map for cell spawns during Route completion
Collect Cells until you can change Zygarde’s form