Disconnect one by one:

the Touchscreen flex cable

the Volume buttons & Mic flex cable

the Dock connector flex cable

Carefully insert a plastic tool under the battery to pry it up from the adhesive. Avoid bending the battery.

Be careful, there are flex cables under the battery. At the last picture in this step you can see the cables and the place were is adhesive tape.

Now you can separate the Dock flex cable.