Option 01: Manual
You could make it using a text editor like notepad. That's at least how we were doing it in 2016. But that process can be painstakingly slow and tricky. If you get a single comma or colon wrong, the entire subtitle file won't work and finding which error is causing the problem is no easy task even when the video is just a couple of minutes. This is the template in which SRT files need to be written accurately up to the millisecond: (the format looks something like this)
00:01:17,440 --> 00:03:20,375 Your subtitle goes here
Option 02: YouTube You use YouTube's Subtitling tool. Instead of the daunting blank Notepad app, once you upload your video, you can use the easier to use subtitle tool to add what text shows up during the video and also download and add it to your Facebook video.
Pro Tip: Use the "auto sync" option to transcribe and let YouTube automatically choose the timing based on how you type it in. Let us know in the comments if you'd like to see a tutorial on that too.
Option 02: Veed.io This one is a paid tool with some features like subtitling also available to free users.