How to use the Waterfall model?

ShreeMBA is quite right in what they say. I would add also that think about your Client and main stakeholders. 

If the Clien tis external, what methodologies do they use/know? Also, have a think about your stakeholders - internally and externally. Do they only know waterfall? Are they open to using a new methodology (i.e. Agile)? If they aren;t, and I have worked with many who are not that open, then it may be best to work in a way that suits them.

Of course, you need to consider your team (Devs, Testers and other Project Team members) and it may be that you are confident to 'hold the hand' of those not use to Agile ways. 

Your Project Manager should really be the one to dictate what methodology is used, rather than the BA, but if there isn;t one assigned to your project, then ask the question in your first meeting with your Client and Stakeholders. Ultimately, whichever you choose is fine, so long as the deliverable is met, you just may need to be more obvious when it comes to reporting project staus (again, the PM should really do this, but if it is down to you then make sure Clients etc know where you are and how you intend to work.).