Hi Rohan,
you need to maintain the BOQ before the project starts so you can do this by maintaining the BOS of the quotation
1-go to t-code va21
2-enter a service material
3-(Go to > item >more functions >performance ) in this screen you can enter your BOQ items .
After you send the quotation to the customer and you awarded the contract you can convert this quotation to sales order and link it with the project "billing WBSE"
Regards,
Islam El-Shamy