A badly done flashback can negatively affect your story. It could create confusion, lose your reader's interest, and affect the flow of your narrative. But, it's usually worth the risk; a well constructed flashback can add texture to your story. It could establish the motives of your characters, deliver information to your readers, and helps the reader connect with your character more.
However, there is a disadvantage in even the well written flashbacks. It already happened and you know the outcome. It loses the much needed suspense on your flashback especially if it's a bad memory. Let's say that your flashback is about a tragic event that involves an armed robbery. It loses all the suspense because the readers already know the outcome, you lived.
Here are a few tips to establish a well written flashback:
1.) Create a trigger or stimulus that starts your flashback
Memories don't come out of nowhere. Unless your story is surreal, it's impossible for that to happen. You have to create a trigger from something in the present to make you remember a flashback. This can make your story have more detail and proves that you pay attention to it. In fact, it can make your flashback be more memorable. Let's say that in the present, you see a knife in your kitchen. This is considered as a trigger and can bring you back to the armed robbery.
2.) Change the tenses in your story
This may be obvious to you but a flashback has happened in the past leading you to using past tense. Assuming your story is told in present tense, this is true. The only problem that arises is if your story is told in past tense. If your story is told it past tense, then at the start of the flashback, use Past Perfect tense. These are words like had done, had ate, and so on. Just do it at the start of the flashback then switch back to past tense because doing past perfect tense on the whole flashback is a little annoying especially on long flashbacks.
3.) Make it full of details but brief
Don't make your flashbacks too long but make them memorable. Don't go on about flashbacks for 20 pages as your reader may lose interest. The same thing happens if you make it too short and don't put any details at all. You have to find a balance between details and length so your reader will remember your flashback.
4.) Lastly, Use flashbacks sparingly
Don't make many flashbacks or else your reader will get confused. Only use them if the only way to give much needed information is through them. Sometimes, the reader will lose interest because it's the primary way to tell the story. They may then get confused and think which timeline is more important thus losing the meaning of a flashback. A flashback must be memorable!
Those are 4 tips and my opinion on the subject. Good luck on your storygame making Zag!