It isn't always easy to pick the right process because the process ID or process name may not be known or ambiguous. If the process has a window and you can see the window title, all you need to do is search for a keyword in that window title. For example, as I am writing the new batch of tips, I could use a one-liner like this if I wanted to identify the instance of WinWord I am using:
Get-Process | Where-Object { $_.MainWindowTitle -like '*tip*' }
Of course, this would require that I saved my document with a title that contains the keyword. Replace the keyword with "Microsoft Word" to find all running instances of WinWord.