You can set a sort order either in the relationship or in a portal. If you set the relationship to sort the volunteer times records by Checking Time in Descending Order, that should do the trick.
I'm new to Filemaker
In that case, I suggest that you explore the documents that you’ll find in the FileMaker Help menu. These are PDFs that are designed to get you using FileMaker. The Getting Started document is terrific. Easy to read and will answer many of the questions that you will have.