What is the best practice of Appian project from gathering requirement to delivery Appian process/es?

It is a BPM best practice to start with modeling the AS-IS process model, check its conformity with business and then move on to modeling the TO-BE process, socialize it with business and then move ahead to granular details.
So at the end of the modeling exercise you should have an L1(level 1 - high level) and multiple L2's depicting all identified sub processes too.
Collecting reporting requirements upfront is important since that plays a major part in designing your cdt's and persistence strategy.

It is a BPM best practice to start with modeling the AS-IS process model, check its conformity with business and then move on to modeling the TO-BE process, socialize it with business and then move ahead to granular details.
So at the end of the modeling exercise you should have an L1(level 1 - high level) and multiple L2's depicting all identified sub processes too.
Collecting reporting requirements upfront is important since that plays a major part in designing your cdt's and persistence strategy.