Congrats for joining the science/religion of cider-making!
I’ll second what
@DBhomebrew stated about SG readings—when and how they are used, and how they are taken. I make 1 gal batches of cider (apple juice is too expensive for 5 gal experiments, for me) and don’t want to keep stealing ~80mL for every hydrometer sample. I also know that a refractometer isn’t as accurate with quite a bit of effort figuring out my specific conversion factor.
I use my hydrometer for my original gravity reading (Using sanitized equipment, I don’t have to worry about any splashing/aeration at the beginning—in fact, aeration at this point is beneficial).
At the three week point, I take a baseline refractometer reading, coating me a few drop of cider. Every week after that, I take another refractometer reading. Each time the process is the same: sanitize all equipment and the outside of the fermenter, open the fermenter and extract a small sample, close the fermenter, take my reading.
Once I get two identical refractometer readings a week apart, I know that fermentation has stopped. My SG readings by refractometer are significantly different than a hydrometer reading would be at this point, but I’m not using the refractometer to determine an accurate SG—I’m only using it to verify that the SG reading is stable.
Once fermentation is done, I usually rack to a secondary vessel and bulk age for a few months. This is optional, with pros and cons on either side.
Just before bottling/kegging, I’ll pull my second and final hydrometer sample. I use it to determine the FG and ABV. (This also helps me determine how much backsweetening is needed, if any.) Even if you don’t care about ABV, it’s a good idea to record OG and FG. This can help you fine tune your process and work towards repeatability.