What are some of the ways you can cue in your reader that your chapter is a timeskip to the past? Let's say you write a chapter just to go back in time and reveal some details about some characters' pasts, how do you cue in your reader without literally telling them there was a time skip?
Use the story
The most organic way to show a broken timeline is to use the story itself to show where in the chronology things happen (as opposed to dates in chapters, or mentions in the narrative description, etc).
Long time jumps
Here are some ways to show long time jumps:
Use a distinct before and after state. Maybe someone is dead after and alive before, so if they are alive, we know it's before. (E.g. the character's parents or grandparents...)
Character age can be used. For instance, if some characters are children in the past and adults in the present they would do different things (go to school instead of work) and may even think and speak differently. (E.g. loudly insisting their secret friend should have a seat at the dinner table and throwing a tantrum if that is denied...)
Characters' relationships might be different in the past and present such as marriages, divorces, friendships, etc.
If you're jumping way back in time, setting and society can also be used to show which time we're in. If everybody is going crazy about the Beatles we're not in 2021 anymore...
Big events can be used to show a before and after. Just to mention one aspect, flying in July 2000 was completely different from flying in October 2001...
Short time jumps
If your time jump is shorter, meaning the general world is the same before and after, you show the chronology, for instance by having plots converge in one or a few scenes. Show the same scene from different perspectives or have one plot show the beginning of the scene and another the end.
You can also use cues like sound, TV news, or other common/"global" information to show what time in the chronology we're at, and thus if your chapter is before, simultaneous with, or after other chapters.
Also, see this question and its answers for more ideas on chronology handling.
First think carefully about just how much of the character's back story is actually important, relevant, and necessary. Often one finds that a tangential mention of a particular event is enough without reliving the whole thing in technicolour.
If you decided you need the whole flashback chapter then:
X/I still remember(ed) the day that... and proceed to tell the story of your past event in third or first person as appropriate. Is an effective flashback structure if you're careful to keep your tenses straight.
The point about the timeskip is to reveal a character's (or situation's, or object's) background - ie, the character was different then. Before the big crucial detail/reveal you're leading up to, think what other details you could include to show these differences - for example in a story about a military general, starting the flashback with 'Private joe bloggs ...' would be an easy one.
As per @ash's answer, keeping a close control over what tenses you're using will be important.
Have each chapter begin with a timestamp. The time stamp should be consistent in format and not the time of day on a clock (I prefer using military time (i.e. 0000-2359, as it saves three characters (a colon and an AM/PM designator)), the date on the calendar, and the calendar year. If you are traversing time zones as well, then make sure you account for the local time of the scene in your time stamp with a designated time zone (most zones will go to a major city in that time zone although it always helps to look up the difference) and make sure you account for the difference in your story's chronology. For example, if chapter 1 is 1200 PST (L.A.) and chapter 2 is 1200 EST (D.C.) then you've gone back in time 3 hours. However if it's 1400 9/7/year EST (D.C.) in chapter 1 and 0300 9/8/year JST (Tokyo) in chapter 2, the two chapters happen simultaneously since Japan is 13 hours ahead of the Easter U.S. (By the way, this discrephancy in dates caused Japan to recieve a charge of a war crime against the U.S. and U.K. in WWII. In international war, surprise attacks. Suffice to say according to Japan, they attacked and declared war on December 8th... but the U.S. maintained the attack was on the 7th and Japan declared war on the 8th, ignoring that U.S. Territory and allied territory was attacked at the same time as Pearl Harbor in the early morning of the 8th.).
You can, a few chapters before the timeskip give the hint of the event which will be covered in the timeskip, somewhat like "this-this happened back then" From dialogue, internal thoughts etc. You can get quite creative here. And when the timeskip does come, you can play the event. This would work when you're going back to the past, from the present.