If you're reading this you either plan to rewatch Made in Abyss and forgot how the watch order goes or you were redirected here by a friend who wants you to watch the show. Whatever the case, here's what you should know:
Currently Made in Abyss has two seasons, two OVAs and three movies.
If you're new to Made in Abyss, I highly recommend that you start with Season 1. Once you're done with the first season, watch Dawn of the Deep Soul. It is the only movie you absolutely need to watch in order to not feel lost when watching Season 2. After you're done with the movie you can skip directly to Season 2, which until we get more content is where the show currently ends.
tl;dr: Season 1 -> 3rd movie -> Season 2
The Other Movies + OVAs
Earlier we mentioned the existence of two other movies: Journey's Dawn and Wandering Twilight. These two movies are recaps of the first season and unfortunately cut out some of the content from it. But on the bright side they add some bonus scenes that were not present in the anime. If you're new to Made in Abyss, it's safe to skip them. But if you're rewatching the series it can be a good way to do so. Watching these is completely optional.
Marulk's Everyday and Together with Papa are spin-off OVAs focusing on supporting characters that you will see in Season 1 and Dawn of the Deep Soul respectively. That being said, Marulk's Everyday can be watched after you've finished Season 1 while Together with Papa can be watched after you're done with DotDS. Keep in mind that neither of these add anything relevant to Riko and Reg's story.
Controversial Content
Contrary to what you may have heard from ignorant social media users and journalists who haven't touched the series with a stick yet still have something to say about it - Made in Abyss does not contain explicitly sexual content or the glorifcation of these topics and under US law is completely legal. The ''controversial'' content in question includes violence, gore, censored nudity and implied sexual assault. Please understand that in no way does the Akihito Tsukushi endorse or glorify any of these actions and that Made in Abyss is simply dark fiction for mature audiences.
Think rationally. If it's available on most of the popular streaming services and your local Barnes & Noble sells the books then it's obviously legal. But if any of the topics mentioned earlier make you uncomfortable, it might not be a bad idea to avoid the series.