What’s your build workflow like?

I’ve mostly built from official sets up until now, but I’ve started looking at MOCs recently and realized the process is completely different when you’re not working from a branded box. For those of you who build this way regularly – how do you usually go about it? Do you design first and track down parts after, or work with what you already have and adjust as you go? Also curious whether people typically go with a single main source for parts, or use a few different ones depending on what’s...