Flashbacks can be tricky and confusing to a new writer. How to construct them is really quite simple. You introduce flashbacks with past perfect tense, then revert to simple past for duration.
For example, “She’d always known they were an item when their eyes had first met.” After that, you would go to simple past, i.e.: “She remembered that first time clearly, as it stood out so dramatically….” Continue is simple past for the duration of the flashback.
Then, when it’s over and you come back to the present story action, use past perfect again. For example, “That moment would always mean a lot as it had signaled from the start that it would be a special relationship.”