Ground planes on 2 layer PCB
Create a copper pour/fill on the top and bottom and stitch them with as many small vias as is reasonably possible. Try to place a GND stitch via next to any signal or power vias you are using to allow return current to flow to the opposite side. Also stitching the entire perimeter of the board while respecting the board houses copper to edge rule is often recommended. Some PCB software has the functionality to automatically fill your board with such stitching vias.