Would like to change an existing outlet to half-hot
with only 2 wires you wont be able to control the 2nd half. If you just want to cut the power to the 2nd half turn the power off pull the outlet and break the tab between the 2 outlets on the red side. if you want to switch the other 1/2 you will need to pull a wire from the red to a switch and then back to the half you want to switch. You should also run a ground from the outlet to the switch.