Pick a clean start date
The easiest start is the first day of a financial year: last year stays in Tally, this year is in the new software. The first day of a month works too, with the year’s earlier months kept in Tally.
Bring masters and balances, not every voucher
What you need is your customers and vendors with their GSTINs, your products with HSN codes and rates, and the balances as on the start date: what each party owes, bank and cash, stock and its value. Re-entering old vouchers adds work and little value; the old books stay readable in Tally.
Check before you switch
After the import, compare the trial balance and the stock value on the start date with Tally. They should agree to the rupee before you make the first invoice.
Run both for a week
For the first few days, enter the day’s invoices in both and compare the totals. When they agree, switch for good.
General information, not tax advice; rules change, so check with your accountant for your case.