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If its a 2 layer board, try to pour ground copper if you can't do that then make sure the traces are thick. Going 4 layers with a ground plane would be recommended but its not necessary for all designs.
if you are routing a one layer board; then you need to make the traces connecting to the ground to be thicker. try to make all unused area in the board as ground copper area. try to make all traces connecitng to gnd as short as possible.
This is a little too broad questions.
-What kind of technology this PCB use, pure digital, Mixed Signal, Analog?
-What is your concern, noise, emi, etc?
-Avoid mixing analog ground with the digital ground, to avoid noise from digital getting into analog circuit. Tie them only at one point. It will be better for EMI to tie both ground in a very big piece as possible but make sure you already seperate aganlog and digital ground.
-For low layer board, pay a lot of attentention to maximize the ground area. Put ground tie vias to get some ground into the area.
Yes, this question is too broad. Actual design depends on what you need to do. Switching power supply will dramatically differ from GigaHertz RF front end. Try to read the books. Good authors are Henry Ott, Mark Montrose, Eric Bogatin, Howard Johnson and other. There are some different approach to grounding, for example star grounding. It is really not easy to chose right grounding and design a good layout.