Okay, this isn’t THE way to do it, it’s A way to do it. If someone else suggests something simpler, listen to them.
I assume you’re on your iPad — me too! Go to Part 1 and tap Edit at the top to go into edit mode, then scroll to the bottom. Then double-click the home button to see your open apps and go to Safari. Open Medium.com in Safari (this is the only way I know to have two Medium pages open at the same time on the iPad) and work your way to one of your chapters (I picked Part 2). At the top of the address bar it will say “A Medium Corporation” — tap on that once, it turns into the URL address, tap again, you should see a Copy option. Tap Copy, double-click the home button, back to Medium (the app), and you can either paste the url in its own line, so that you get a box
Or type “Part 2”, select the text, tap the Link button just above the keyboard (the URL you just copied fills in automatically), tap Link, and you can go to Part 2.
Hope that helps! 😊 (It really is a good story that deserves to be read!)