Dear Tanu,
Before hiring any consultant, it is better to obtain proposal for the consulting services from him/her. Has the consultant clearly specified the parameters of the consulting services? In response to the proposal from the consultant, you may issue Purchase Order (PO) to her. In this PO, specify clearly the terms and conditions under which the consulting services to be provided. You may add additional clauses about procedure to pay her fee, credit terms, data security, non-disclosure of information etc.
Thanks,
Dinesh Divekar