Make sure you gather and capture as much requirements as possible, do not get too much worried about the format of the document or the structure, the focus should be more on completeness rather than tidiness or presentation. Involve the end-users or customers as much as possible during this phase. Make sure you record all the spoken and unspoken requirements, try to focus on what rather than how.
In this phase make sure you clarify the requirement with the SME or users, developing prototype or test cases can help you clarify the requirements, make sure that the requirements are measurable, this phase is very important as it also determined the completeness of the requirement by formally inspecting the documents as a group.
In this phase evaluate and agree which requirements will be done first, this phase will help you to remove those high cost but low value features. In this phase project scope will be identified and milestones will be defined.