I need to add custom fields like customer GSTIN to my sales invoices. How can I do this?

To add custom fields like customer GSTIN to your sales invoices, register. You need to follow the below steps:
Step 1: In the default setting of standard format GSTIN already coming in Sales Invoice,
Step 2: If it's designed, you can modify the invoice format by contacting the dealer.
Step 3: They will help you create a custom invoice design that includes the desired fields.
or if you want to add the GSTIN in the sales invoice register.
Step 2: Go to Display, then Account Books, then Account Registers (standard), then Sales Register.
Step 3: After that, you will find the option of Sales Registered to be shown for all sales types and one sales type, then select the relevant option.
Step 4: After that, you will find the option of sales registered for all parties, groups of parties, one party, and selected parties, then select a customer for whom you want to see the report.
Step 5: After that, you will find the option for a date range, then type the relevant date range and select relevant information if required. Press Y on that particular window.
Step 6: Set Y to show the party GSTIN or TIN number to display the GSTIN filed, and then execute the report.
Step 7: After that, you will find your sales registered for all invoices with GSTIN filed.