A GST tax invoice has required contents. Most guides list them as a checklist, which is accurate and not very useful at a counter. This one goes in the order you actually fill them.
Before the first invoice: the things you set once
Three things belong to your business rather than to any single sale, and setting them properly once removes most of the per-invoice work.
Your own details: legal name, address, GSTIN, and the state you are registered in. That state is what decides the tax split on every invoice you will ever issue, so it is worth checking rather than assuming.
Your invoice series: a sequential number series for the financial year. Sequential matters. If you bill from more than one counter, each counter needs its own series rather than two counters racing for the same next number.
Your product list with HSN codes and tax rates, so the line items do not need looking up while a customer waits.
Step 1: the customer
For a B2B sale, you need the customer's GSTIN and state. For a consumer sale, name is usually enough. The state matters even when the GSTIN does not, because place of supply drives the tax split.
Step 2: the line items
Each line carries a description, HSN code, quantity, unit, rate and the applicable tax rate. If a line has a discount, it belongs on the line rather than being mentally deducted from the total.
Step 3: let the totals compute
Taxable value, then the tax split, then the invoice total. This is the step that should never be manual. Hand-calculated tax is where rounding differences creep in, and rounding differences are exactly what reconciliation catches later.
Step 4: the fields people forget
Place of supply, if it differs from the customer's address. Whether the sale is under reverse charge. The amount in words. A signature or the standard note that the invoice is computer-generated and does not require one.
Individually these look like formalities. Collectively they are the difference between an invoice your buyer's accountant accepts and one they send back.
Step 5: issue it, then leave it alone
Once an invoice has gone out, it is a document of record. If something needs changing, the correct route is a credit note, not editing the original.
This is the single most common informal habit that causes trouble later: correcting a bill by opening it and changing a number. It breaks the audit trail, it can leave stock and ledger figures inconsistent, and if the original was already reported, the corrected version no longer matches what was filed.
What good software removes from this list
Steps 3 and most of 4 should be automatic. Your own details and invoice series come from setup. The tax split follows from two recorded states. The amount in words is derived. What is left for a human is the customer and the line items, which is genuinely all that varies between one sale and the next.
That is the test worth applying to any billing app: how many of these steps does it still make you do by hand?
B2B and B2C are not the same document
Selling to another registered business means their GSTIN belongs on the invoice, because that is what lets them claim credit on the purchase. Selling to a consumer, it does not apply.
The practical consequence is that asking every customer for a GST number wastes time, and never asking loses your business customers their credit. The question belongs at the point where you record the customer, once, not at every sale.
Reverse charge, briefly
On certain supplies the liability to pay tax sits with the recipient rather than the supplier. Where that applies, the invoice has to say so explicitly, which is why it is a field rather than an assumption.
Whether it applies to anything you sell is a question for your CA. What matters here is that the invoice must be able to state it, and that leaving the field unmarked when it should be marked is a defect in the document rather than a formatting preference.
A mistake found immediately versus a mistake found later
These are genuinely different situations and it is worth knowing which one you are in.
If the invoice has not left your counter and has not been reported, the pragmatic answer is usually to cancel and reissue, keeping both in the record. If it has gone to the customer, or if the period has already been filed, the correct route is a credit note against the original.
The wrong answer in both cases is opening the saved invoice and changing a number, which is common enough that it has its own article below.
How Vanij handles this
Vanij fills your business details, the invoice number, the tax split, the amount in words and the required declarations from setup, leaving the customer and the items as the only per-sale entry. Corrections go through credit notes rather than edits.
See also CGST, SGST and IGST explained for the tax-split rule, and GST invoice generator for what the finished document contains.