Hi Naba,
You need to issue ‘bill of supply’ to the customers since you have opted for composition scheme. Following details are required to be mentioned – 

1. Name, address, and GSTIN of the taxpayer & customer,
2. HSN Code or Accounting Code,
3. Document number and date
4. Description and quantity,
5. Value of goods and services,
6. Place of supply (including the name of state),
7. Delivery address (if it is different from customer’s address),
8. Signature of authorized representative

You also have to mention a line “The supplier is supplying goods under Composition Scheme.”