One of the challenges of good world building is consistency, making sure all the details you cook up will work plausibly together. See, it isn’t just about assembling copious amounts of background history, mythology, character biographies, etc.; it’s also about making sense.
Ever hang three gibbous moons in the sky because it’s such a cool image? Then make sure you aren’t writing about a seafarer becalmed on a gentle ocean. Three moons equal chaotic tides and weird currents. Shipwreck is likely.
Or maybe you’ve designed a society with members that advance in status and position through regular assassinations. Interesting, unless the protagonist is gregarious, frequently throws lavish parties, invites strangers to partake of his hospitality, and doesn’t employ a bodyguard or a food taster. Is he a fool? Probably, but he’d better not live to a ripe old age.
A story world is made believable through consistency, vividness, specificity, and accuracy. In the previous two paragraphs I’ve shown you examples of inconsistency.
You achieve vividness by using specific terms rather than vague generalities. Here’s vague: The desert was hot. It looked like a big, brown wasteland.
Instead … the wastelands were an anvil, the sun a hammer beating on Bara’s skull. Every breath scorched his lungs. Thirst swelled his tongue, but with his waterskin hanging empty on his belt, he had nothing to drink except what perspiration he could lick from his upper lip. Tottering across sand that burned his boot soles, he peered at the wavering horizon without hope of reaching it in time ….
Whenever possible and appropriate, incorporate the physical senses (taste, touch, hearing, sight, smell, and magic) because they will provide a compelling sense of place.
Accuracy comes from research. If your characters carry swords, determine what kind they are, including whether they’re designed for slashing or stabbing, and what their length and weight are. Do you know anything about swordplay? Learn the rudiments. There are all kinds of guides, books, and Internet articles on weaponry, footwork, and combat skills. Better yet, talk to fencers and watch their bouts. Don’t rely on what you see in the movies. Hollywood sometimes mixes blades, hilts, and fighting styles for bigger spectacle.
And if your characters ride horses, for pity’s sake go to a public stable and rent an animal for an afternoon. Ride it. Learn how to saddle and bridle it, how to get on and off properly, how to groom it, and how to feed it. Inexperience and unfamiliarity will always show in your written copy, no matter how hard you try to fudge.
If you’re an urban dweller, spend a few hours in a park or drive outside the city to the woods. Draw in the scents and sounds. Feel the slight temperature variance in the air. Distinguish between the rustling of a tree canopy in the breeze and the furtive stirring of a mouse in the brambles. Listen to the sound of your footsteps over dead leaves, or muddy ground, or twiggy undergrowth. Then go home and bring it alive on the page.