Dear Anupam

• use 10ng per 100bp of purified product for big dye Sanger sequencing
• Purify your PCR product in order to remove unincorporated dNTPs (left over from your PCR reaction) primers and salts
• This can be done by gel extracting your band from agarose gels or alternatively running 10% of your PCR product on a 1% agarose gel and having confirmed a single specific band purifying the remaining 90% directly without running on a gel e.g gel extraction =
• https://www.qiagen.com/gb/shop/sample-technologies/dna/qiaquick-gel-extraction-kit/#orderinginformation
• Direct clean up = 
• https://www.qiagen.com/gb/shop/sample-technologies/dna/qiaquick-pcr-purification-kit/#orderinginformation
• Alternatively you can treat your PCR product with shrimp alkaline phosphatase (SAP) and exonuclease III (Exo) and then ethanol precipitate your treated product
• This enzymatic procedure inactivates PCR dNTPs and degrades your primers respectively: 
• https://www.eeb.ucla.edu/Faculty/Barber/Web%20Protocols/Protocol5.pdf
• You cannot simply precipitate your PCR product as this will lead to contamination of sequencing reaction with both primers and excess dNTPs

Thank you Rajendra sir and Laurence sir