Purchase, download or otherwise obtain a copy of the schematics for a bass octave pedal. There are a number of free schematics available online that allow enthusiasts to create pedal "clones" or other circuits that approximate the effects of commercial pedals. Experienced tinkerers can modify a schematic or components to customize the pedal, or even build one from their own schematic.
Read the schematic and record each necessary component and its specifications, if this information was not already provided in a list. Decide whether to use the components in the schematic or make modifications. Purchase the components needed, or scavenge them from old electronic devices.
Drill holes in the pedal casing where the important components will go including the battery, PCB board or integrated circuit, knobs, jacks, and any additional switches or LEDs. Some pedal casings come with several holes pre-drilled for these main components. Once the pedal casing is set up with proper holes, paint or finish it as desired. You will not have the opportunity to paint it after the components have been put in.
Install the battery compartment in the bottom of the pedal casing for quick accessibility. Mount the PCB board or integrated circuit board inside the casing, and connect the positive and negative battery wires to the corresponding leads on the board -- soldering each connection. Solder the pedal switch in the casing to the PCB board or circuit. Using the schematic for reference, solder any additional circuit components to the corresponding leads of the circuit, mounting any protruding components -- potentiometers, switches, jacks or LEDs -- internally before soldering the connection. Insert a battery. Test the circuit with a live signal once everything is connected.
Bolt the pedal casing closed, and mount any additional external pieces such as knobs, jack protectors or the pedal's rocking component -- the pedal itself. Plug it into a live setup, and test the variable settings on the knobs or switches. Use a simple bass tuner to determine if the circuitry is correctly scaling your pitch one octave. Any additional modifications may be made by opening the case and carefully modifying the circuitry while the pedal is disconnected and unpowered.