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The UK publishes a guide to supporting documents for a visitor visa here https://www.gov.uk/government/publications/visitor-visa-guide-to-supporting-documents/guide-to-supporting-documents-visiting-the-uk
There is nothing you personally can do to βensureβ that her visitor visa application is approved. It may help you to read How to prove that you have significant ties with family in your home country? and What details should a good cover letter contain to back up a UK Visit Visa application?