Opening the book…
A reader will believe in a dragon if you first make them believe in the mud. The fantastic becomes credible not by insisting on itself but by sitting inside a world so concrete in its ordinary details that the mind, having accepted the small true things, extends the same trust to the large impossible one. This is why the best fantasy is often so grounded, so full of weather and hunger and aching feet: the mundane is the collateral that secures the loan of the marvelous. Pile wonder on wonder with no ordinary ballast and the reader floats free, believing nothing; anchor the wonder in the felt texture of daily life and it lands with the weight of the real.
Surround your impossible elements with concrete, sensory, ordinary detail, and let characters react to marvels the way real people react to real things, with fatigue, boredom, greed, or fear rather than constant awe. Ground the fantastic in bodily reality: the dragon smells, the spell leaves you shaking and starving, the spaceship's toilet is a nuisance. Spend your most specific, physical writing on the mundane, and let the wonder be described almost plainly, so it borrows the credibility of everything solid around it. When a marvel rings false, add mud, not more magic.
High romance, myth, and deliberately dreamlike work may reach for a heightened register where mundane ballast would only weigh down the intended lift. Some stories want the reader unmoored. But for most fantastic fiction, the impossible is bought with the ordinary, and a world that skips the down payment tends to bounce.