This watch guide is intended for anyone who wants to keep up with the plot and most exciting parts of Steven Universe, while skipping the silly/fluff parts near the beginning. If you enjoy the fluff, this probably isn't for you.
The episode numbering is weird on a number of services because the episodes are so short (11 minutes) and they get combined into two-part 22-minute episodes. So I've given the episodes you should watch by name; double-check the episode name when you're watching them.
There are 25 key episodes to watch, which I've double-starred (**) and put in bold. Those episodes should total about 4.5 hours.
The 25 double-starred episodes are the bare minimum "get all the plot points" episodes. If you enjoy the show, I recommend going back and watching the single-starred ones too, of which there are 10; they add valuable character development and depth. Or if you have an extra couple hours, watch them upfront with the rest.
After Season 1, if you like the show, I'd recommend watching all the episodes in Season 2 and beyond.