I would like to try an answer that overlooks the contentious term 'subtext'. Regardless of its meaning (or meanings) in literary theory, the truth is most amateur writers (in the sense of writers who write for the love of it and discover the techniques as they go in order to perfect their craft) misunderstand a lot of literary terms. I studied literature and remember quite a few terms which were difficult to grasp correctly even with the teachers explaining them and correcting faulty interpretations.
So when writing a story, at what point do you plan for these items [mirrors, symbols, and subtext]?
Do you have a separate thread in your mindmap or outline alongside the main thread in the outline form?
Write the whole thing and work in the subtext in the third draft? Is there a particular point in the process when it's easier?
IMHO, it depends. If you're writing literature (with the elitistic capital L), it should be mostly already part of the plot, with the characters representing something bigger than them and every word and action being carefully thought out to create the right impression. I'll assume that is not what you're doing.
So you have a story with the plot, subplots and characters fleshed out and ready to navigate the action. But you want to make all this deeper; you want to shine a subtle light on a less visible personality trait (because the villain is tragically in love and refuses to acknowledge it while the hero is hiding from the reader and the narrator how dark his heart truly is).
You have two ways to go about it:
- if you're a planner, go back to the plot and decide where you can drop little hints and what is the nature of those hints.
Do you want the plot to represent politics, moral values, human growth? If so, make sure the plot conveys these ideas without contradiction (unless that is the point of your tale) and perhaps not too obviously. If you want your hero to be the spiritual saviour of a community, don't have him die on a cross in order to do so. Subtlety is the key word here.
Do you want to jab a criticism somewhere along the story but not have the story be all about that criticism? Work it into a subplot.
Do you want to use universal symbols (e.g. the red rose as symbol of love) or do you want to create your own symbols, which will stand as such only for this text (e.g. the oak leaf as a symbol of love because it reminds the character of when he met/lost/whatever the love of his life)? If the latter, decide how that symbol came into existence within the tale.
Do you want to use a wealth of these items and create a highly symbolic, metaphorical tale? Be careful. Personally, I find that works better in extremely well crafted Literary works. If that is not what you want, then, again, plan how many of those items will be used per chapter on average and balance their levels of subtlety. Be particularly careful to not use these items as themselves but weave them into the plot and actions in a way that they will almost be invisible.
- If you are not a planner, play it by ear. That's mostly my case. I have a generally vague plan and then discover these things as I go. If, say, I notice that my subplot could be used to criticise whatever, then I may stop to revise it and make sure it fits with the criticism I'm aiming for.
May I give an example of a historical novel (a telling of real people and events which have a lot of unknown gaps in the history books) I'm working on with examples for 'non-planners'?
The scene: There is one high-noble woman that is dealing with the fact her marriage has been annuled and she's now a hostage locked in her chamber. She is thinking of her father, condemned as a traitor, and wondering what fate will the daughter of a traitor have.
The thought process: Traitor! Who is the biggest traitor of all? Let's have her sit by a tapestry which is a tryptic with the crucification in the middle and Judas hanged on one lateral. Ah! At the time, the crucification scene showed the Virgin Mary fainting by the cross. So let's have the character sit at the feet of Christ (because she is the pawn whose father will sacrifice quite a few times to reach his goals) and let's have her think of the scene while praying for Christ's and his mother's intersection. In later chapters we can now learn the character's mother (who had a long attested history of depression associated to health problems) did die shortly after this blow to her daughter.
The plot points:
1. the noblewoman goes out with a hunting party to hunt water fowls to advance a certain plot point.
2. the nobleowoman will later have her husband be involved with a court lady nicknamed 'egret neck' (it sounds much nicer in the original language)
3. the noblewoman's maid will try to avenge her cheated lady
The thought process:
They're out hunting egrets already! Let's have a subplot where the maid kills an egret to avenge a slight towards her lady (it killed the lady's hawk; egret's were known to do that). This will foreshadow the 'egret neck' lady (who, spoiler ahead, does end up being killed) and show the reader how this maid is so loyal to her lady that she will avenge her even when petty slights are involved.
This approach is messy and can require going back and forth to introduce, change or cross out items and scenes.