I'm trying to watch the documentary on the making of Kingdom Come: Deliverance, and I want to watch it with English subtitles. I see that there's a download folder for downloading subtitles. But how do I make the subtitles appear in the movie?

Depends on the player you are using. Just load the subtitles file into it. Drag and dropping the file onto the player window is usually enough. If you specify what media player you are using, I can give specific instructions.

You download a ZIP file. Extract it, find the MO_KCD_ENG.srt file and load it into the player. Make sure you have subtitles enabled/shown in its settings.

Sorry, I'm not real smart lol. In my GOG.com account, I click on the subtitles folder. A zipped subtitles file appears in my list of downloads. Then I click on the download file for the movie itself in my GOG.com account. The movie starts playing in my browser. But the subtitles don't appear. I don't even know where to find the settings for the player, if I'm using a player at all

Are you watching the movie in your browser? I didn't even know that was possible.

I would suggest using K-Lite (Media Player Classic).

https://files2.codecguide.com/K-Lite_Codec_Pack_1605_Mega.exe

I've been using this for years, and it's regularly updated.

Just rename your subtitle file to match your movie title:

Movie.Name.X264.mkv (movie file example)
Movie.Name.X264.srt (subtitle file example)

I hope this helps.

That's the way to make it work for most movie players (even my TV) - simply place movie and subtitle file in the same directory and have rename them to the same name (file extension aside).

I prefer VLC media player.
I have no problem with any movie and subtitle with it. Simple and portable version is available.

I'd go with CCCP (Combined Community Codec Pack) which includes MPC. Been using it 10+ years. Depending on the player. You have to basically extract the files into the same folder, and the player if it detects subtitles (identical filename except extension) it may automatically enable regardless of your preferences.

In MPC you can press S for Subtitles and it will cycle through the ones (built-in or otherwise) while VLC you may have to rightclick and change the subtitles after you load it. I tend to use VLC when there's multiple audio tracks and MPC may try to play all at the same time. (Or when i want to speed up the video at less than 2x speed, good for listening to news or lectures at 1.3x speed)