Documents needed in term of testing methods

You asked 

....what all documents would we need in term of testing for the said methods? for example, do I need to validate/ verify these methods?

Yes, you do need to validate all test methods that are going to be on your scope of accreditation. You need to meet requirements of ISO 17025 clauses 7.2 and 7.6 for this activity

You would need to include at least all mandatory procedures and records.
See the Whitepaper Checklist of mandatory documents required by ISO 17025:2017
For more detail on what is requires, read the whitepaper Clause-by-clause explanation of ISO 17025:2017
Both are available for download from https://advisera.com/17025academy/free-downloads/

You asked How?

The ISO 17025 toolkit includes the procedure for validation and verification of methods, named Test and Calibration Method Procedure, along with two supporting documents Test Method Development, Verification and Validation Register and Test Method Development, Verification and Validation Record. The procedure is also available separately at https://advisera.com/17025academy/documentation/test-and-calibration-method-procedure/

The overall techniques for method validation are listed as well as the required records. It is the responsibility of the laboratory to choose the suitable technique and performance parameters, plan experiments and use suitable calculations. This is not in the scope of the toolkit. It is important to reference sector specific guidelines and meet specific regulatory and accreditation body requirements.