what are the steps to disconnect a simple circuit?

Answer 1

Depends on where you want to disconnect it.
You can turn the switch off as suggested. However if it not a switched circuit, then plug a lamp into a plug in the circuit you want to de-energize. Turn off the breakers one by one until the lamp goes out. The entire circuit is now off.
If you are going to work on that circuit is is always a good practise to test the wires for voltage prior to opening any switch or receptacle box. An inexpensive tester is available hardware stores, Home Depot etc.

Answer 2

Flip the switch to the off position.