Locate and shut off the power to the circuit that will feed the track light. Use a screwdriver to access the existing fixture junction box. Remove the two plastic twist caps attached to the black and white wires feeding the existing light fixture. Remove the light fixture and set it aside.
Follow the manufacturer's instructions for attaching the track lighting track to the ceiling. Run the track wiring up through the bottom wiring hole of the track and up into the light fixture junction box. If the junction box is located away from the track light, then do the following: Place a section of ROMEX® cable from the track light over to the junction box; at the track light end of the ROMEX® cable, strip 1/2 inch of insulation off the black and white wires. Then, connect the two white wires together and then connect the two black wires together. Cover both splices with plastic twist caps.
Go to the junction box and connect the white wire of the track light to the existing white wire in the junction box. Locate the black wire that comes from the switch at the junction box. This could be a single black wire or a single white wire that has a black mark or black tape on it. Connect the black wire of the track light to the wire that comes from the switch. Place a twist cap on that splice. Turn the power on to test the light.