Moschoula (the owner of the property) was incredibly nice, very helpful, and all around fantastic. The apartments are on the main drag (essentially the only street) in Platis Gialo, and right about in the middle of town. The apartment was clean and comfy, and had a nice little balcony where we could enjoy our coffee and also let our towels and swim suits dry.
The beach offers little natural shade, and several beach-front restaurants offer use of their sun beds and umbrellas for 5 euro (or more), whether you're eating/drinking there or not. So it's a good thing Moschoula has FREE reclining beach chairs and umbrellas you can use!
IMPORTANT NOTE ABOUT LOCATION: the location shown on the hotels.com app is different from that shown on Google maps, and neither are correct. As you're coming into town, find Notos Cafe (on the left/beach side of the street) and go one to the next building. The sign isn't very obvious, but you should be able to find it. Fortunately, Moschoula answered when I called her and she came out found me while I was lost wandering the street.
Really, I just can't say enough. It was a great experience, and for the price you really can't beat it. If I'm ever back on Sifnos I will be staying here again.