My project was to make a smash as NAND gate.  Since I was a beginner, I choose a simple circuit.
First, I started with literature survey.
I searched everything about my circuit in google.
Next was to implement it using esim . 
Esim was working properly. My design also showed correct output.  
Next was to install zip files of sky130 and implement using ngspice. This was the biggest task for me. 
I was not getting the write output.
Library files were not found.
Then I added 'x' before each mosfet M1,M2,M3 and so on.
Finally i got output. But it was having huge error.
I tried to reduce my error by changing the width and length of the mosfet. Finally I got the exact output.