To begin with it's always best to test your original consoles sound works. It isn't always required, as you have have a dead speaker, but if your console works first its a good sign.
Open up the Game Boy Advance using a Triwing screwdriver by removing the 6 screws on the back, and 1 more philips screw in the battery compartment.
Once inside, it is easier to also remove the main circuit board from the shell by removing the 3 philips screws from the centerline of the motherboard (one to the left side, 2 to the right side above power slider switch. Before lifting out the console, push the tabs on either side of the ribbon connector at the top towards the top of the shell. This releases the pressure on the ribbon cable and you can pull out the cable, leaving the screen stuck down in the front shell. Now remove the circuit board from the shell.
Solder the dehum kit to the pads shown.
Be careful when soldering the Dehum kit pads that you do not short the pads together or remove the resistors or capacitors near the pads.
Solder the bottom right pad to the bottom of the CP1 capacitor pad, the bottom left pad to the ground spring of the battery terminal, the top right pads to the bottom of the small and large capacitors. The top left pad does not need attaching to anything.