Dear @Aman,

In order to design a mask you have to define the physical lay out of the device by drawing its cross section and its plan view showing its different layer structure.

There are computer aided lay out tools which are two D drawings tools.

One of the programs that can help you making physical lay out is called Lasi: https://lasihomesite.com/

Examples of different transistor lay outs can be found in books of integrated circuits such as VLSI DESIGN TECHNIQUES ...., BY GEIGER https://www.amazon.com/Techniques-Circuits-McGraw-Hill-Electrical-Engineering/dp/0070232539

After making the lay out drawing you can form them what is called the photomasks. If you have small volume you can use photomasks other wise you can make a working metal mask by process of photolithography.

Best wishes