Hi Anish,
As a Ariba Partner, you mostly do all the requirements gathering, designing, completing the custom field/process templates that you can get from Ariba Representative on the project. Actual field creation, conditions building, eforms, d-forms creation is done by Ariba Rep(s). Regarding process templates in upstream you can take up the template creator role and build the process templates for contracts and sourcing if you are well versed with the same else you can fill the excel template and Ariba rep(s) can build it for you. So you can generally say that the partner does the design and Ariba (Consulting/Support team) does the build in an on-demand world. There exists some grey area where you have to take a call on who does what and clearly define it in the roles and responsibilities of the project plan and ensure all parties are in agreement.
Regards
Vilas