Use the order by operator

Completed

Sort the rows of the input table by one or more columns.

The order by operator can utilize any column or multiple columns by using a comma separator. Each column can be ascending or descending. The default order for a column is descending.

SecurityEvent
| where ProcessName != "" and Process != ""
| extend StartDir =  substring(ProcessName,0, string_size(ProcessName)-string_size(Process))
| order by StartDir desc, Process asc