This bit of grammar seems quite useful. I picked it up from japanesepod101.com’s upper intermediate 3rd season. It’s a way of stating a prerequisite to avoid some negative outcome. どんな時どう使う日本語 provides the following illustrative example sentence:
When eating raw vegetables, if you don’t wash them properly first then pesticides are a concern.
Breaking it down a little:
~てから means “after ~ [verb in て form].
でないと means “if not,” so all together it’s something like: “if not after ~” followed by some negative verb or unwanted result.
With that, I can make my own example: