Ask a CA what they need and the answer is usually "your data", which is accurate and completely useless if you do not already know what that means.
Here is the practical version, and more usefully, what makes the job slow when it goes wrong.
Sales, completely
Every sale for the period, with the invoice number, date, customer, taxable value and tax. The word doing the work there is every.
A CA can work with almost any format. What they cannot work with is a set that is missing some sales, because there is no way to know it is incomplete from the inside. It looks fine until it is reconciled against something else, by which point the period is closed.
Purchases, which people forget
Sales get sent; purchases get remembered on the last day. But your purchase records are what determine input credit, and a missing purchase invoice is money you paid tax on and did not claim back.
This is the single most common way small businesses lose money quietly at filing time, and it is entirely avoidable by recording purchases when they arrive rather than hunting for them monthly.
Credit notes, as their own list
Returns and corrections are separate documents and belong in the hand-over as separate documents, referencing the invoices they correct.
If a return was handled by editing the original bill instead, there is nothing to hand over, and the sales figure quietly disagrees with what was actually collected. That is its own article, and it is the habit CAs complain about most.
The invoice series is the first thing they check
Numbers should run in sequence with nothing missing. A gap raises an immediate question: was there an invoice there, and where is it?
Usually the answer is innocent, a cancelled bill or a test entry early on. But every gap costs a conversation, and if you bill from two counters sharing one number series, you will have overlaps as well as gaps, which is a considerably longer conversation.
GSTINs on B2B invoices
A business customer's GSTIN needs to be on the invoice at the time it is issued. Adding it afterwards, when their accountant asks, means amending a document that has already been reported.
The habit worth building is asking once, when you first record that customer, rather than per sale or not at all.
What actually makes their job slow
It is rarely volume. A CA can process a great many transactions quickly if the set is coherent.
What slows things down: gaps in the numbering, invoices edited after issue, missing GSTINs, purchases arriving in three batches over a week, and figures that do not tie out between what you say you sold and what the documents show. Each one is a question, and each question is a message thread.
Send it in one go, and not on the last day
Filing has deadlines and your CA has other clients with the same ones. Material arriving in pieces, late, competes with everyone else arriving in pieces, late.
The deadlines that apply to your business depend on your registration type and turnover, and they get revised, so those are worth confirming with your CA rather than assuming. What is always true is that earlier is cheaper in attention, and sometimes in fees.
Ask for the format they want
Some CAs want a spreadsheet, some want a specific export, some are happy with PDFs. Asking once and then sending the same thing every month removes a recurring negotiation.
It also means the monthly hand-over becomes a two-minute task rather than a decision, and tasks that are decisions are the ones that slip.
Keep your own copy of what you sent
Whatever goes to your CA, keep the same set yourself. Not because you distrust them, but because six months later the question "what did we file for August" has to have an answer that does not depend on someone else's inbox.
This matters most if you ever change accountants, which happens more often than people plan for. A business that holds its own records changes CAs in a week. One that does not spends a month reconstructing its own history from someone else's files.
The habits that do all the work
Almost everything above reduces to four small things done at the time rather than at month end.
Record the sale when it happens, not from memory later. Record the purchase when the goods arrive, not when you are looking for it. Ask for a business customer's GST number the first time you serve them. And when something has to be corrected, correct it with a credit note rather than by opening the old bill.
None of these takes extra time in the moment. All four remove work at month end, and between them they eliminate most of the questions a CA has to come back with.
How Vanij fits
Vanij produces a GSTR-1 style export along with HSN-wise and tax-summary views, keeps invoice numbering sequential per counter so two billing points cannot collide, records purchases against suppliers, and handles corrections through credit notes rather than edits.
See also how to create a GST invoice and GST billing software.