Manage a Flashback and Return
Without confusing your reader.
The theory is simple.
End the previous paragraph with a lead-in to the flashback, preferably the last sentence of that graph.
If the flashback is long, start a new chapter. If not, return-carriage to start a new scene.
End the flashback with a lead-in to the action interrupted by the flashback; again, preferably the final sentence of the flashback.
The execution of the practice might take some time to wrangle. (Then again, you might grok it right away.)