Remember FL Studio works off of PATTERNS.
A pattern is just like a “group of sounds” that you can add into the playlist to start building your arrangement so you have full control of when and where it plays.
I like to use my number pad – and + buttons to switch between patterns.
If you open one virtual instrument, right-click piano roll, and play your chords, that becomes Pattern 1.
You’d then go to Pattern 2, open a new virtual instrument, right-click it and go piano roll, then you can play a melody, which is now unique, and in its own pattern.
Now in the Playlist, you can add both Pattern 1 and Pattern 2 for you to have full control of your arrangement by adding and removing the patterns!
At this stage you want to focus on if you’re in SONG MODE or PATTERN MODE (top of FL Studio.. right by the stop, play, and record transport buttons). You’ll most often want to be in song mode, which plays the Playlist (pattern mode only plays that one pattern if you want to focus on it for a moment).
So think in terms Patterns, and try to only put the instruments pertaining to that pattern into 1 pattern only.
For example, if you made a percussion loop with 3 sounds, that can be in 1 pattern.
If you have a piano instrument, I’d say that only goes in 1 pattern so you can add/remove it at any time.
There’s no “rules for patterns”, but just remember that the less you have in a pattern the more control you have in your song’s arrangement.