Writing Longer Flashbacks

Question

In my story my MC has suffered a severe injury and has no recollection of who he was before it occurred. He knows what happened and where he was when it occurred as he was told by the people who found him and nursed him back to health.

I want a large portion of the plot (and world building) to be done through his flashbacks. While not immediately relevant to the story, it will become the final act. Any ideas on how to write longer flashbacks?

For Example:

• When he goes somewhere he visited before and has a memory of the place, but unlike Deja Vu he doesn't feel it.
• He has dreams of battles, or conversations with (in the future) key players in the socio-economic make up of the day.

I was thinking if it were almost a chapter-esque length, write the chapter in italics, and if was longer make it clear to the reader by segmenting the read with a ------------- separating the present from his mind, and typing in different text or italics. I want it to be fairly uniform between the longer memories and the shorter.

Is there a common practice?

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Edit:

While this question, which was pointed out to me by Cyn (I missed it apparently during my initial research); is quite similar, it deals with a lot of dialogue which can guide that OP's MC through the memory/flashback and through conversation it can be apparent to readers, which time period the text is referring to.

Whereas my issue and desire to implement the flashbacks is more situational (e.g. MC approaches a fortress or a valley, and remembers a snippet of his life-before, often without context to him). For the portions of his life/world that is remembered through dialogue that question is hugely beneficial, but not so much for the situational flashes, or dreams.

Answer 1

I am not aware of a "common practice" - writers are fickle beasts who tend to disregard rules. But there's nothing that says the progression of your story needs to be "linear unless marked otherwise".

The first example that comes to my mind is The Dispossessed by Ursula Le Guin. First chapter of the novel - the MC makes a dramatic escape from his homeland to another country. Second chapter - the MC is a very young child. The chapter is not in italics or anything like that. It's clear that the progression from chapter 1 to chapter 2 isn't linear, pursuant to the fact that one doesn't jump from being an adult to being a baby. Readers are smart, they can figure it out.
From there, the tale proceeds on two parallel timelines: one explores the MC's life after leaving his homeland, the other - his life before. The character's age and the entirely different situation provide the cues the reader needs to position themselves.

Making time-line jumps chapter-breaks is useful: it's a useful way to mark a transition for the reader. Especially once you've established the fact that such transitions occur, and there may be little else to mark it. It's also useful to establish early on that flashbacks are something that happens in your story, that is that it's not all going to be linear. In the aforementioned example, Ursula Le Guin sets it by jumping to a point where the MC is a child. This makes the transition obvious. Later on, when the MC is an adult in both timelines, she relies on the fact that the transitioning back and forth has already been established.

In addition to jumping to a sufficiently-distant point in the past, you can establish that "we're not where we were a moment ago" through switching to a different POV, through recounting it in present tense (though that has some additional effects on how the reader would perceive the passage), or through otherwise making a significant change from what has just been established. (For example, if main story-time is in summer, your flashback can be into winter.)