Tolkien left behind less how-to advice than most novelists of his stature, and what survives is scattered across one essay and thousands of letters. The thread running through all of it is world-building. He believed an invented world has to obey its own rules before a reader will step inside it, and that getting those rules right is the real craft.

The Direct Answer
Build a world consistent enough to believe in, and the reader will step inside it and stay.
He called this making of a believable imaginary world 'sub-creation,' in his 1939 lecture On Fairy-Stories.

Sub-Creation and the Secondary World

Tolkien's clearest statement on craft is the lecture he gave in 1939 and later published as On Fairy-Stories. In it he argued that a storyteller is a kind of maker, a sub-creator who builds what he called a Secondary World. The test of that world is whether it holds together, however strange it looks. You can put a green sun in the sky, he said, but making readers believe in it takes real labour, because everything else in the world now has to behave as though a green sun were normal.

The story-maker proves a successful 'sub-creator'. He makes a Secondary World which your mind can enter. Inside it, what he relates is 'true': it accords with the laws of that world.

J.R.R. Tolkien, On Fairy-Stories, 1947

The lesson for any world-builder is that consistency does the heavy lifting. Middle-earth feels solid because its distances, languages, genealogies, and histories all agree with one another. Tolkien worked those details out even when no reader would ever check them, on the theory that people can always feel when a world is thin. Belief is fragile. One contradiction and the spell breaks.

He drew a firm line between this and mere invention. Anyone can pile up marvels. The harder job is discipline, holding a world to its own logic so tightly that the marvels stop feeling arbitrary. That's why so much of Tolkien's effort went into things readers barely notice: calendars that work, family trees that add up, languages with real grammar. The wonder on the surface rests on all that quiet bookkeeping underneath.

Start With the Names

His practical method flowed from the same idea. He built languages and names before plots, and he trusted a good name to carry the seed of a story. He once described a tale as something that grows like a seed in the dark out of the deep leaf-mould of everything a writer has read and loved. You don't assemble a world from a checklist. You let it compost, then grow. That's why his invented history feels discovered rather than designed.

It also explains the appendices. Most novelists would cut a hundred pages of genealogies, timelines, and notes on pronunciation. Tolkien printed his, because to him they weren't extras. They were the roots the story had grown from, and letting readers glimpse them made the whole thing feel older and truer than any single lifetime could invent.

For how this played out day to day, see the full J.R.R. Tolkien writing routine.

What You Can Steal