Let me illustrate how NOT to do it:

Picard faced Q across the length of the glade. "Why have you brought me here, again?"

"Why?" That insolent smile ignored his anger. "You persist in misunderstanding me, my dear Captain. I bring you here because I want you here. And, because I can."

Picard remembered the first time he'd found himself in this impossible landscape of Olympian trees and whisper-soft grasses. The sun had been shining that day, too. On a confusion of flowers and what had to be butterflies sailing in the light breeze. But where he would have expected to see a riot of color, the landscape had all been in shades of gray. Q had been dressed as he was now, the red shirt that mocked Picard's uniform before it had been replaced with this drab tunic.

They had been having the same, or a very similar, conversation that day.

"We are in this dismal place so I can remind you of my abilities," he remembered Q saying. "My power."

"I am reminded," Picard had told him. "Will you take me home now?"

"As soon as you admit what I am to you," Q had said arrogantly. "As soon as you admit what we both know. That I am important to you. That you . . . feel for me."

That had been the one thing that Picard had been determined not to say. And he hadn't said it. Not then, and he wouldn't say it now. "You know how I view you," he had told Q calmly. "You are a menace to the safety of my ship and my crew. That is all."

"Admit that you . . . like me," Q had demanded. "That of your own free will, you have come to care for me."

Q had wanted him to say that Q's power over Picard's unforced heart was as real as Q's power over the physical universe. But he hadn't said it. He hadn't even thought it. He had been able to resist Q then and he resisted him now the same way. Today Q asked the same question, and Picard gave him the same answer.

"I am prepared to admit nothing of the sort. Now, and for the last time, will you return me to my ship!"

Yuck. Okay, that's enough. I assure you, I wasn't trying to write that badly, it just came out that way. It's the natural way many of us write. Without thinking. And there's nothing wrong with writing without thinking during your first draft. The key to a first draft is to get the idea down on paper (on-screen) while it's fresh in your mind, right? It's what you do next that makes the difference.

Anyhow, you probably don't have to read that snippet twice to realize that I jump around in time between the flashback and today's scene, that I try too hard to explain (or 'show') what Picard is feeling. You see a lot of flashbacks written this way because the author's mind is constantly moving back and forth between today's scene and the one the flashback is supposed to illustrate. The author is trying to draw connections with a huge, black marker, instead of giving the reader the courtesy of letting them think for themselves.

Inevitably you're going to find yourself slipping into Past Perfect (PP) tense. And your readers will be snoring in about four paragraphs.

Past Perfect tense - also called Passive Voice. The word "had" is your biggest clue that you're writing in Passive Voice.

had found had shone had been had announced had wanted

There are times you'll use PP tense and it will be quite appropriate. But you'll use it sparingly. Just occasionally. You want to spend most of your time writing in Active Voice. That's the mode where things happen. The voice that gets the reader into your story, involved in your action, and caring about your characters.

PP is an improvement over bouncing around from tense to tense, but every use of a verb in PP just reminds the reader that this is a digression, it's not the real story. Which is not what the author should be wanting. If I'd written the above passage entirely in PP, it would look like this:

Picard faced Q across the length of the glade. "Why have you brought me here, again?"

"Why?" That insolent smile ignored his anger. "You persist in misunderstanding me, my dear Captain. I bring you here because I want you here. And, because I can."

Picard remembered the first time he had found himself in this impossible landscape of Olympian trees and whisper-soft grasses. The sun had been shining on a confusion of flowers and what had to be butterflies sailing in the light breeze. But where he would have expected to see a riot of color, the landscape had all been in shades of gray. Q had been dressed in a red shirt that mocked Picard's uniform before it had been replaced with the drab tunic he had been wearing when he appeared in the glade.

"We are in this dismal place so I can remind you of my abilities," Q had said. "My power."

"I am reminded," Picard had told him. "Will you take me home now?"

"As soon as you admit what I am to you," Q had said arrogantly. "As soon as you admit what we both know. That I am important to you. That you . . . feel for me."

That had been the one thing that Picard had been determined not to say. And he hadn't said it. "You know how I view you," he had told Q calmly. "You are a menace to the safety of my ship and my crew. That is all."

"Admit that you . . . like me," Q had demanded. "That of your own free will, you have come to care for me."

Q had wanted him to say that Q's power over Picard's unforced heart was as real as Q's power over the physical universe. But he hadn't said it. He hadn't even thought it. He had been able to resist Q then and he resisted him now the same way.

"I am prepared to admit nothing of the sort. Now, and for the last time, will you return me to my ship!"

Not much of an improvement, I know. Marginally better, because once the reader is in the past, they stay there until the end of the flashback.

The best thing you can do with a flashback, is avoid reminding the reader that they're in one. Use one transitional sentence at the beginning, and the end, to get the story in and out of the flashback, then use present tense all the way through the scene you're describing. That makes the action seem more real, more immediate, and more important. It keeps the scene alive and your reader's attention on the story.

Picard faced Q across the length of the glade. "Why have you brought me here, again?"

"Why?" That insolent smile ignored his anger. "You persist in misunderstanding me, my dear Captain. I bring you here because I want you here. And, because I can."

The glade had looked this way that first day, as well. An impossible landscape of Olympian trees and whisper-soft grasses. The sun shining on a confusion of flowers and what had to be butterflies sailing in the light breeze. And yet, it was all wrong. Because where he expected to see a riot of color, this landscape was all in shades of gray. Q wore a red shirt that mocked Picard's normal uniform. Not that he was wearing it now, because Q had replaced it with a drab tunic.

"We are in this dismal place so I can remind you of my abilities," Q said. "Of my power."

"I am reminded," Picard sighed. "Will you take me home now?"

"As soon as you admit what I am to you," Q added arrogantly. "As soon as you admit what we both know. That I am important to you. That you . . . feel for me."

The one thing Picard was determined not to say. "You know how I view you." He used the unemotional tone he knew worked best to calm his strange persecutor. "You are a menace to the safety of my ship and my crew. That is all."

"Admit that you . . . like me," Q demanded. "That of your own free will, you have come to care for me."

Q wanted him to say that Q's power over Picard's unforced heart was as real as Q's power over the physical universe. But he wouldn't say it. He wouldn't even think it. He had been able to resist Q that day, and on all the days that followed, and he resisted him now.

"I am prepared to admit nothing of the sort. Now, and for the last time, will you return me to my ship!"

Okay, it's never going to be a great scene. But notice, as I said, the way I use the same tenses inside the flashback that I would be using to describe a present day scene. And notice how casually the introduction into and exit from the flashback part is handled. No abrupt shock to the reader.

Better yet, ask yourself if you really need a flashback? Can't you write today's scene, and just allude casually to the previous one(s) without leaving your main story?