I haven't settled into a uniform technique yet. (I'm still learning a lot about writing!)
But with SB3 I outlined the crap out of it. Then filled in my outline with more detailed outlining, rinse and repeat. Then, once I knew what was supposed to happen and what details had to be covered in each scene, I started writing the dialogue.
These days I leave a LOT of sound cues out, because I have the luxury of not having to trust another director to insert the right sounds. So I keep half of the sound cues in my head and explain things to actors as we read through it.
Using the free "Celtix" script writing software for the first time with this script and I love it!
But for SB1 I put a TON of SFX description and cues all over the place.
Long story short... I don't really write for other people, so I'm not much good for advice on this one. Sounds like you're off to a good start, though! Have fun!