Hi i have very big table in rows more than 100 million and also many check and forien key constraints
planning to create non clustered index, what is the best practice to do it on existing table.
what should be keep in mind

Have you performed the analysis to ensure that this Nonclustered index will help the performance?

If so it is better to perform the task during a low activity period on this database, to ensure no overlap of other jobs are in place.

If it can't be helped, it can't, can it?

If everyone agrees that the index is beneficial, and the change is approved, inform your business users couple of days before that and when this is going to happen and what the implications may be. Remind them on the day it is going to happen and then do it during a low(er) activity period.

The reality is that many of the decisions are situational, although some of the decisions should be approached from a best practices perspective then evaluated as they pertain to your specific environment. 

Although many best practices do exist, application, business, technology, resource and time constraints can pose significant challenges to address the need, which is the fastest data access possible.