No formal process.

If I'm considering the indexes from missing indexes DMV, first I'll make sure there are no partial redundancies within the index recommendations. I'll also make sure that there are no existing indexes that I can widen.

Once that's done, I'll get all the queries that run against that table (from the metadata if all access is via SP, or from profiler if they're not), run them, get their current performance characteristics. Create the new index, or alter an existing one, run the queries again and see if there's been an improvement.