i always outline my scripts, even for my short films. and when i outline, i always use omni outliner. it’s a great piece of software.
for the current script, i made 3 columns: one for the main story beat, one for the slugline (location), and one for any additional notes, like act breaks. i’ll start with just the bare information, like inciting incident, midpoint, climax, etc. then i’ll slowly add story beats, move stuff around, flesh it out. i also color coded it so i can quickly see the ratio of A-story beats to the B-story beats.
once the outline is solid, i’ll switch to movie magic screenwriter and actually write with the proper format.
there’s this other program called movie outline which looks similar but with a couple more features specific to screenwriting, but that costs $200 where omni outliner costs $70 for the professional version and $40 for the standard. plus omni outliner is more flexible so you can use it for other things and work the way you want. i’m a big fan and i recommend you try it out.