They are pretty good - although they tend to assume quite a bit of knowledge of audio and computers (especially regarding terminology).

[Note: my Audacity version is pretty old, so things may not be in the same exact place as described below.]

Under the Help menu (keyboard shortcut Alt+H) select Quick Help. From the left side navigation, choose FAQs. Scroll down to the heading Recording - How To's and click the Can I play a track while recording a new one on top of it? link. That gives the bare minimum instructions on how to set up the software, but includes a link to the Tutorial - Recording Multi-track Overdubs with further info.

Those help files are also online here:
https://manual.audacityteam.org/man/faq_recording_how_to_s.html#overdub
https://manual.audacityteam.org/man/tutorial_recording_multi_track_overdubs.html

You might find their forum more helpful (https://forum.audacityteam.org/index.php), someone might have already asked the same questions that you have. There is also a basic wiki page (https://wiki.audacityteam.org/wiki/Audacity_Wiki_Home_Page).

It sounds to me like you just need to overdub your new part while listening to the previous one and then clipping and/or adusting their respective volumes as necessary. You may need to slightly edit the delay/latency depending on your software settings and your hardware. Do a couple test recordings following their overdubbing tutorial as a guide and play around with the outlined settings to see if you can get your setup streamlined for your needs. If you get stuck, try searching for and/or asking specific questions on their forum.

I didn't read your prior thread, so I don't really know what recordings you are making on your computer. And I haven't really used Audacity in quite a while, although I can probably remember the basics.

Once you get things setup, recording new tracks should be quick and easy - however, you may need to figure quite a few things out to get to that point. The software is very popular, so Google searches should also yield results.

Good luck.