Remove the access panel from the piece of equipment, and locate the volume and tone controls to be combined into a single concentric dial. Use electrical tape to secure the group of wires running to each control into a little bundle. Draw a diagram of the contacts to which each colored wire is soldered. Each control is likely to have positive, negative and ground wires.
Examine the contacts to which the wires are soldered, and check for any symbols or marks that denote whether each contact is the positive, negative or ground. Usually, the convention of red for positive, black for negative and green for ground is used, but this can vary.
Heat up the soldering iron and ready the work space. Clear any materials that could be damaged by the heat. Melt the solder connecting each of the three wires to the existing volume control. Pull them free of the contact terminals, and clean off the excess solder using the pump. If there is too much old solder, slightly cut the wire back, and strip off some new insulation. Be careful to leave enough length to reach the new concentric control.
Solder the positive, negative and ground wires to the contacts on the new concentric control. Allow the solder to cool. Wrap each contact in a small quantity of electrical tape, covering all exposed metal. Repeat this process with the tone control. Remove the trio of wires from the existing control, clean them, and solder them to the new control.
Remove the locking nut around the base of the control that's to be replaced with the concentric. Push the control back into the cavity and remove it. Push the shaft of the new concentric control through the same hole, and secure it with the locking nut as before.