Raise GST invoices for contracts and chargeable calls, and record the payments you receive.
Open Billing → Raise invoice. Choose Contract (AMC) or Chargeable service call and the record. The description and taxable amount fill in. The GST rate comes from Settings. The dialog shows the CGST/SGST split, the total and the invoice number that will be used.
Click Save and download PDF. The invoice is saved, the PDF downloads and the next invoice gets the next number.
Record payments. On an invoice row click Record payment, enter the amount, date, mode and reference, then Save payment. A part payment lowers the outstanding total by that amount, and the invoice stays on the Outstanding tab as Part Paid until it's paid in full.