The following explanation comes from using Merriam-Webster’s Rhyming Dictionary.
For those who write metered, rhyming poetry, a rhyming dictionary will give many more word choices than most people can think of on the spur of the moment. Seeing what rhymes will also trigger other word associations and send poets off in other directions.
Poets who write free verse aren’t looking for end rhymes, but often choose words with close rhymes elsewhere in their lines. Other sounds and shades of meaning are important, but a rhyming dictionary can give a memory boost.
A rhyming dictionary groups words by end sounds instead of alphabetically by beginning letters. Words ending in the “an” sound, such as wingspan, are followed by words ending in “ana,” such as banana.
Within each end sound listing, words are grouped alphabetically by number of syllables. For instance, one-syllable words which rhyme with “an,” such as ban, bran, can, clan, etc., are in the first group. Two syllable words, such as adman, ashcan, bedpan, began, etc., are in the second group, followed by three and four syllable groupings.
The end sounds in headings always start with a vowel sound, such as “an,” “ating,” or “ation.”
Remember that words will be grouped by a simple spelling of the sound, not all the actual spellings. Example: words rhyming with night will be listed under “ite.”
Some spellings produce different sounds, such as Aswan vs. Batman. Pronunciations are given after each bold sound heading, and will head you in the right direction.
For simple rhymes, start with the ending syllable sound, such as “un.” For more complex rhymes, try several syllables: for skating, try “ating” instead of “ing.” For ethnicity, try “icity” instead of “itty.”
Just as you wouldn’t choose a word from the thesaurus without knowing the different shades of meaning, be particular when looking for a rhyming word for your poem. It must fit the meter, the emphasis must be on the right syllable, and it must still convey exactly what you want.
For children’s poets, be sure that the meter is perfect and the rhyme is exact. Concentrate on the concrete words in the list, instead of abstract. Make sure the words you choose are within the tone of the age for which you are writing.
For poets not needing a perfect rhyme, the Oxford Rhyming Dictionary is another good choice. It groups words by how closely they rhyme. Mat/cat are more perfectly rhymed than rancho/poncho or orange/lozenge, for example. But because it's a British dictionary, organized by pronunciation, Americans many have a harder time with it.