You need to collect/verify below documents from your customers.
- Sales and Purchase Files
- Debit Notes & Credit Notes
- Declaration against sales
- Sales & Purchase Register
- Purchase Summary as per returns.
- Sales Summary as Per Returns.
- Sales Returns
- Fixed Assets Bills
- Sales Reconciliation
- Purchase Reconciliation
- Profit & Loss A/c & Balance Sheet
- Income Tax Audit Report
Along with above you need to take care of some activities like Verification of all Purchase and Sale bills, Verification of Debit and credit notes , Checking of declaration and L.R. , Verification of Other income in the Profit and Loss A/c , Verification of addition to fixed assets, Preparation of Main sheet , Preparation of Sale and Purchase summary, Tax Position and Vertical Calculation , Completing Reconciliation of Accounts, Preparation of Annexure I and IV , Preparing Data Sheet., and Preparation of Form 704.
Please let me know if you need any more assistance on this or if you have any other queries.