How does someone remember everything, And all necessary details? For each sin?
the nature of the sin, number of occasions (if known), and gravity of the sin must be mentioned,
In part, it depends on how frequently one is going to confession. If one is confessing regularly, as we’re encouraged, the extent one is having to search one’s memory would not be so extensive, as is the case if one is confessing for a span of years, as the original poster’s circumstance of first confession with RCIA envisions.
If one is asking how one practically remembers what they wish to confess…well, I will admit, having been a confessor for decades and myself having gone to confession long before I was a priest, that I always have a list in the palm of my hand of what I want to say. All I have to do is go in and tick off the list and not arrive to “Erm…what was that last point I wanted to say?” But that’s me.
If, on the other hand, you are asking how to remember what needs confessing…that is why a preparation aid, such as an examination of conscience, and the basics, such as reading and knowing the Catechism of the Catholic Church or other good and solid texts on the spiritual life, are great resources.
It really is serious sins, also called mortal sins, that must be confessed, and then with the addition of number, at least approximate. Venial/lesser sins, if mentioned at all, can be mentioned in passing and without details. “I told white lies since my last confession.”
Personally, I generally find that one can have the intimation that one should be more specific when specificity is called for. To use a favorite example from when I taught moral theology, most women would know that there is a distinction between “I committed murder” and “I committed uxorcide” (I killed my husband). I won’t comment that there are those who would see it as more serious and there are those who would see it as less serious or even excusing of culpability.

(There are all sorts of interesting canonical implications of uxorcide that made it a particular favorite to turn to with inquisitive first year moral theology students being introduced to the interaction of moral theology and canon law).
It is like the classic in the old catechisms of “I robbed” and “I stole the proceeds of the parish poor box”. The latter adds the sin of sacrilege to the sin of theft – and I think most people would intuit that robbing the poor box that is inside the church merits the extra seconds to make a special mention of it in confession.
The question is: does the detail really affect the nature of the sin such that it compounds the sin and/or makes it more grave or conversely mitigates or excuses the presumptive guilt of sin? Obviously, a married woman should not confess an instance of adultery, if what occurred is that she was knocked unconscious and became the victim of rape.
Or, returning one last time to my earlier “I lied under oath”…the addition of “because my wife was being held hostage under threat of death until I gave this testimony, which would vindicate an accused criminal” completely changes the culpability imputed for the perjury.
I fear pursuing any of this further takes us too far from the original poster’s query, although the explanation of when to qualify the sins one is confessing may be useful for a first confession.