It's easier to apply it after painting/finishing and before you install any wiring/pickups etc.

Cover the inside of the cavities and the underside of the pickguard. On a Tele, you'll need to run some grounding wire from the main control cavity to the neck pickup rout. I normally just copper tape the ends down.

You just make holes in the copper tape to push the wires through. You can't help it, but try and keep any holes as small as possible. Run the copper tape for the neck pickup rout over the edge on to the top, so that it can make contact with the tape on the underside of the pickguard. Run the copper tape for the control cavity up over the edge on the two ends of the cavity so that it makes contact with the metal control plate (but cut it so that it's all hidden by the control plate.

Run the copper tape up over the front and rear edges of the bridge pickup rout so that it makes contact with the bridge plate. You'll be running a ground wire for the bridge, so the bridge will ground the pickup cavity copper. Or you can run a ground wire from the control cavity to the bridge pickup cavity, copper tape it down, and let the copper ground the bridge.

Always run the copper tape right up to the top of the cavities, and slightly over the top where you can without it showing. You want to make as complete a copper/metal enclosure as you can with the smallest of holes in it.

Use a multimeter to test for continuity of copper tape grounding once finished. And check that the control plate makes ground continuity with the bridge plate.

Just beware of guitar signals grounding on the copper tape. It's worth sticking some insulating tape on the bottom and sides of the control cavity to prevent pot and switch lugs touching the bottom/sides. Also the jack cavity is another place where the jack socket signal contact can touch the copper shielding when a jack is inserted.