Here's my shot. I tried a few methods and ultimately decided on a sandwich of Focus Magic and USM plus some 'spotting' and tonal tweaks. I didn't bother cropping to 16x20 or resampling up. That last should be done for final print prep but that's a discussion for another day.
I would suggest cropping to 12.5x16 and not 16x20. You don't have a lot of information here, you would be wasting a lot of original image and it would look better smaller. You would also be presenting it in an aspect ratio closer to the original. See the bottom image.
I've sat here for the better part of half an hour thinking how to respond to this and I'm not coming up with any answer that provides help. Sharpening, is an arcane art better left to Voodoo practitioners.
It involves capture sharpening ( from RAW or in-camera Jpeg). base sharpening while processing, and output sharpening for print...keeping in mind the specific printer that will be used. Much of that is determined by experience, personal taste and a decent set of digital tools.
I haven't used any sharpening help in many years. Maybe someone else here can point the way to your first steps.
If you want to get into it into great depth, try 'Real world Image sharpening' a book from Jeff Schewe/Bruce Fraser but its probably not where you should start.