Step 1: Re-do hotbars on every job. And I mean every job. Everything has changed heavily since ARR (no more TP for melee Jobs, for example.)
Step 2: Pick a couple jobs to focus on. The MSQ without the Road to 80 bonus (more info regarding this bonus here) is good for 2-3 Jobs maximum.
Step 3: Do the MSQ. Everything is gated behind progressing the Main Story. Note: Depending on when you stopped playing, you may not have done the Crystal Tower 24-man raid series. That's now a required part of the MSQ, so it's worth doing
Step 4: Side Content. If you feel like taking a break form the MSQ at any point, then you can look into unlocking side activities they've added or unlocking optional content like Raids, Trials, etc. Note: The Crystal Tower 24-man raids are now a required part for the MSQ (players before a certain point in the MSQ are forced to do it when they reach a certain point, otherwise it locks you out of Shadowbringers if you're past that point in ARR).
They've made it really easy to identify which side quests are more important. Quests with a Blue '+' Icon often unlock additional content (like jobs) or features/activities.
if you haven't already, definitely look at doing the Hildibrand questline when you're getting a bit bored, it'll add some much needed comedy relief to some of the seriousness of the MSQ (the first quest in the chain starts with Wymond in Ul'dah at level 50).