These are the best places to start learning Mando'a, including my recommended order of using them.
Reference material
Dictionary (includes parts of speech and nice sorting features; also includes select fan words): https://mandocreator.com/tools/dictionary/index.html
Original grammar guide (covers only the basics): https://web.archive.org/web/20110713135503/http://www.karentraviss.com/html/grammarguide.htm
Lesson plan
Introductory lesson
This 19-word course covers the most essential words, which you can sprinkle in your conversations right away. As a bonus, it includes audio, so you can even start speaking a little Mando'a. Note that you'll need to log in (so it can track your progress) and the course can only be accessed from a browser.
First set of lessons
For a strong foundation, I recommend beginning with the Mando'a Lesson's "Teaching Mando'a" playlist.
Second set of lessons
After watching all of the "Teaching Mando'a" playlist, I suggest moving on to this Memrise course of 94 words, created by one of the first fan teachers of Mando'a (DiscWarrior). Note that you'll need to log in (so it can track your progress) and the course can only be accessed from a browser.
With it, you'll have enough words to start sprinkling in Mando'a into texts and posts and to begin creating very simple sentences. I suggest posting sentences in one of the Discord servers linked on the Hibirar page. to check that you are doing it correctly. Otherwise, you'll give yourself bad habits that are hard to undo.
The most common mistakes that learners make:
1. Not conjugating the active verb. The active verb is the one that tells what the subject/actor in the sentence is doing. If you keep your sentences to one subject and one verb, then that verb is always the active verb.
2. Not knowing the difference between different parts of speech. In English, one word could be used in three or four different ways — in the same sentence, if you're being funny! In Mando'a, that is rarely the case. Nouns can never be used as verbs. Verbs can never be used as adjectives. Conjugated verbs are never nouns. English and Mando'a share a lot of similarities (even if you're sticking to the base dictionary and grammar guide), but they also each have their own quirks.
There are other mistakes that often have to do with trying to take on too much at once, but those two are the biggest ones.
Third set of lessons
After mastering the "Mando'a for Beginners" course, I suggest moving on to this one, which is most of the over 1100 words from the original dictionary divided into categories. This will show you almost the extent of what Mando'a can talk about. (Remember that it was designed as flavoring for novels, not as a substitute for any Earth languages.)
If you want to mix up your learning, I suggest adding in the "Speaking Mando'a" playlist from Mando'a lessons. When available, have the Mando'a subtitles on, so that you can follow along with what Shoya is saying. Each video is very short.
Translate the Mando'a into English, if the English isn't given on screen. Since there is no Mando'a-only dictionary, it is much harder to do full immersion.
Fourth set of lessons
If you want to learn more about Mandalorians along with Mando'a words, listen to Mando'a for Di'kute. It is less structured than the Mando'a Lessons videos, but provides lots of context for Mando'a words. Each episode is about 60-90 minutes long, though if you're paying attention and want to take notes, you'll need to block out more time than that to study with them.
My biggest ding on the content is that there are mistakes every episode (sorry, An'u), which they usually (though not always) mention and correct in the next episode. However, they do not list corrections in the episode descriptions, so I recommend listening to the opening 10 minutes or so of the next episode, to check for corrections for the previous one.
Additional lessons
If you want even more ways to learn Mando'a, the Hibirar page has even more options. https://sites.google.com/view/hibirarmandoa/bajur/hibirar