I use Lightroom. I've been going through and cropping to a 4x6. I hope it works
What software do you use?
One thing to remember is to not give the cheap places anything but sRGB files to print from as they don't use anything else in their fully automated Fuji or Gretag printers so the colours will be all wrong otherwise.
You will need to either crop to the desired aspect ratio or use a border with the correct aspect ratio. WalMart and the other cheap printing options crop a bit, so using an added border will probably give more predictable results. Your standard files are 2:3 in aspect ratio, 8X10" is 4:5 and 5X7" is 5:7.
Sent from my SM-G950U using Tapatalk