The first method I'm not sure is correct. But yes, you are correct on the second. Unlike other eye techniques, this one activates based on need. When your emotions overwhelm you and your brain tries to meet the need at hand and forcefully progresses to the next stage. The Sharingan is not something that can progress by simply thinking about your past, but activates based on the present needs. When Sasuke fought Naruto, Naruto was moving so fast that Sasuke's then two tomoe Sharingan could not track Naruto's movements. His three tomoe activated in response to this need of tracking Naruto or else he would have lost the fight. He obviously activated three tomoe and we all know what happened next. You were also really vague when you said "flashback" and "the process" and "somehow". Though you were on the right track, that's not specific enough. It is not the actual flashback itself but the emotions produced by it, that activate the Sharingan then cause it to evolve. Most flashbacks are used to build emotion behind a technique not to activate a technique itself.
Show me that you need the three tomoe. Not that you just want it. Come at me.