Well, one can make it up as one goes along, and indeed there's a whole strand of criticism built around the idea of revisionism, where we reconsider literary works in the light of chances that have occurred since they were written to gain new perspectives on meaning and interpretation - a trivial example would be reading older poems that include the word 'gay' (as in carefree) reinterpreted in the modern context where it implies homosexuality. Everyone knows that that secondary meaning was not and could not have been in the author's mind when the poem was composed, but the facts that we can't 'un-know' the modern meaning of 'gay,' and that it is possible for such revisionist reinterpretations to be meaningful or even moving despite the demonstrable absence of authorial intent to create such meaning tells us something important about the nature of the reading process, and thus of the thinking process that underlies it.

That said, symbolism generally is constructed deliberately. When I write fiction it's almost impossible for me to start at the beginning and proceed to the end - I start out with fragmentary ideas of momentary images, snatches of conversation and so on, and traverse them in sort of random fashion until the outline of a sequence emerges which suggests a plot. Various symbols will suggest themselves along the way, and one or two may 'click' by encapsulating or paralleling the story structure particularly well. Once I've identified one of those then it acts like structural reinforcement, so I'll go back and add it or emphasize it in various parts of the story where it seemed to fit well.

The symbol(s) you pick in turn become a source of new ideas for solving other plot/structure problems. So suppose I have a story about some lonely woman and I realize this person likes flowers (I didn't decide this, I just kept picturing her eating dinner alone but always with a vase of flowers on the table so I'll roll with it), and then it hits me that flowers have a short life and wilt and die so that's a good metaphor for my character's situation if she doesn't find love. This isn't a full story yet, though - why is this person lonely? Dunno, don't have any ideas...well I'll think about flowers a bit....vases...you need to water the...do you pick them or buy them...will they attract insects....oh wait, flowers use insects to pollinate, what if my character was stung by a bee and unconsciously displaced a fear of sex onto the incident? Now I have a story because I have a person in conflict with herself, impelled to seek love by the fact of time's passage but afraid of the emotional or physical risks that entails. Hmm, not only does my lead like flowers, but has become obsessed with them - she owns a chain of florists or she's a master flower arranger that supplies flowers for all the best weddings, and has put all the energy that normally goes into a romantic relationship into floral activities instead. Then she meets someone who's perfect, but maybe hates cut flowers, which is a problem because my florist is obsessed with having everything just so and needs to learn to accept other people as they are naturally...well now I have a whole little world here with numerous interconnected symbols and some obvious characters and situations that I can develop, and the name of the flower store and the story is 'Perfect Bouquet,' and it seems like it's going to be a romantic comedy.

Exploring and exaggerating the semantic associations of the initial floral symbol has provided me with a basic plot and a handful of obvious scenes that I can begin writing immediately. Most books on writing say things like imagine your characters, find out everything about them and flesh them out in detail, then put them together and see what they do...' I guess that works for some people but I think it's total crap. I don't know any florists or much about flowers, apart from a summer working for a commercial flower supplier when I was young (which was basically a warehouse job). But taking a generic person, an emotion, and finding some symbol that can embody that were enough to get the ball rolling.

You can do this in reverse too. Sticking with the romantic comedy genre, take You've Got Mail. Meg Ryan is a proto-hipster who owns a tiny local bookstore, Tom Hanks is the ruthless CEO of a bookstore chain that wants to put her out of business, and of course they have to fall in love (via the internet, which ironically has since put a large bookstore chain out of business - see my point above about revisionist readings, because it's interesting to ask whether Nora Ephron even contemplated that possibility when she wrote the film).
Now the story structure is pretty generic - it's sort of Beauty and the Beast set in a bookstore, and BatB is in turn a metaphor for mating strategies. But why bookstores? Couldn't Meg Ryan have been a plucky neighborhood grocer being squeezed by the opening of a new Walmart store, or a feisty buggy whip maker fending off an encroaching automobile salesman. But that she sells books rather than vegetables or equine accessories suggests she's primarily contemplative and maybe sensual, as opposed to nurturing or dynamic, that she's a collector/curator rather than a consumer or maker, and so on and so on.

The process of exploring these semantic associations isn't a scientific one, and you're absolutely right that it can degenerate into horse shit that ends up telling us more about the critic's inner life than the work that is supposedly under scrutiny, but the inquiry is itself informative. Why is Jaws about a giant shark rather than a giant alligator or bear? It wouldn't be hard to imagine a story about a village near a swamp or a forest that was menaced by the appropriate animal and have the plot unfold quite similarly - creature features are among the oldest kinds of stories and thus among the most formulaic, but we don't mind because they're so relatable, as long as the personality of the Creature is developed sufficiently well. Symbols work like icebergs; the large part that is submerged (below the level of consciousness)serves as a foundation for the relatively small visible part: the authorial design is to present the reader with something whose significance is easy to underestimate but which will become clear during the development of the story.