Short answer: fill both GSTINs, the invoice number and the line items, choose intra-state or inter-state, then download. Tax is computed per line in paise, so the CGST/SGST columns and the grand total agree to the last paisa.
What makes an invoice a tax invoice
Both GSTINs, a unique serial number, the date, HSN or SAC codes, the taxable value, the rate and the split of tax. Miss the HSN code or the buyer GSTIN and an input-credit claim on the other side can be rejected — which is why those fields are on the form rather than buried.
Numbering discipline
Invoices must be numbered serially within a financial year. Pick a scheme — INV-2026-001 — and keep it; a gap or a repeat is the kind of thing an audit notices. This page prints whatever you enter and does not manage the series for you.