Idle fluctuate or idle surge?

I Received a call about 6pm today from a fellow engineer to come over and check a car and rectify the problem. I asked the car owner as well as the engineer what was done previously to the car before the problem set in. He said that he just bought the car not long ago and it was consuming excess fuel. The catalytic converter was removed, thus the net was used for the construction in place of a new catalyse. The Engineer also recommended servicing, which he later did. Unfortunately, after cleaning the throttle body and reinstalling it the problem. The idle was fluctuating please how can I solve the problem? What should I do to bring the idling of the car down to steady 1,000 RPMs again?