The first thing that is needed is a description of what the circuit is supposed to do, with parameters as to what is available with which to do it. Once that's in place, the circuit designer will produce the schematics. The easiest way to get the schematics is to ask the circuit designer for them and the next step after getting the schematics is to put the circuit together.

Is something missing here?

THANK YOU;

for instance take it this way.Am going to design a potentiostat for an electrochemical cell for more channels.I already have the basic design for the limited channels.

I need to extend the existing design so that it can work for more channels, for which I need the schematic design.

now my problem is how would I know what should be done to make it work for more channels.?

is there any guide ?

This is why you are in school. TO LEARN how to design circuits. A design may, or may not exist.

The easiest way to get a schematic is to design the circuit.

Asking others to do your work won't get you very far in life.

What is needed for Circuit Design

1. Your text book(s) & relevant course / education

2. A problem to solve

3. Your ingenuity; ingenuity => engineering, using the information, education, and available tools & resources to solve a problem.

fm!