Before you proceed, here's the riddle I came up with, through composing this answer. Obviously, the answer = spoiler follows close behind, so if you want to try to guess it, do it before proceeding below the line.
Eight hours of death. Beneath your head -.
a colored box that's full of dreams.
It listens to your tales unsaid.
Soft gifts of birds escape its seams.
Start from the end: the answer. Think about your target audience - if it's a kids' riddle you won't give them a riddle with 'Euler's Constant' for an answer. Make sure it's not too obscure, unless you're aiming at a very specific audience.
Let us try something nice and simple. Oh, out of top of my head, since I'm feeling sleepy, a pillow.
Pick a set of associations which can describe the subject, themes realated to it.
- feathers/down
- sleep
- soft
- cloth
- bed
- under head
- square
- pillowcase
- dreams
- container [contains feathers]
Turn some or that into either metaphors or unclear synonyms. Think of ways of bundling some of them, possibly unrelated. Try to have the metaphors related.
If you have difficulty coming up with metaphors, see my crude recipe from Writers.SE although it's always better to craft these by hand.
Craft roughly half the required number of them for now. I'm aiming for a four-liner, so two will do.
square + container + dreams = a box of dreams
feathers - fluffy; taken from birds - fluffy gift of birds
Now is a good moment to start looking for rhymes. Professional poets will wince, but I cheat with rhyming dictionaries. RhymeZone has fewer hits but of higher quality. Rhymer finds much more but some of them are quite poor.
Dreams:
1 syllable:
beams, beam's, creams, deems, eames, gleams, hiems, nemes, reames, reams, reims, schemes, screams, seams, seems, siems, steams, streams, teams, team's, teams', teems, themes, weems
2 syllables:
basim's, esteem's, extremes, hakeem's, hakim's, kareem's, pezim's, racemes, redeems, regimes, regime's, supremes
3 syllables:
joachims, joachim's
Let's look for something promising. Oh, a pillow has seams. Now, what can we write about seams? Sometimes the fall apart, sometimes feathers come out through these. We already have a line about feathers, so let's consume that one. Synonyms, extra adjectives, to pad for rhythm, and we have two first very nice lines.
Colorful box that's full of dreams
Soft gifts of birds escape its seams.
Now, since we consumed the 'gifts of birds' we need to resupply our store of metaphors to fit to half of the remaining lines. So, one. Similar noun, difference between them and the metaphor is ready.
Sleep - death - 8 hours. "Eight hours of death." Tell me if that's not a nice metaphor.
Another visit to rhymezone and... nothing promising. "Breath-death" is totally ridden into ground. Let's pad with something at the end instead. Straight from the list above, 'head'.
"Eight hours of death under your head".
1 syllable:
bled, bread, bred, dread, dred, dredd, ed, fled, fread, fred, freda, pled, schwed, sffed, shred, sled, sledd, sped, spread, stead, swed, szwed, thread, tread
2 syllables:
abed, behead, embed, imbed, instead, misled, misread, numed, purebred, retread, sayed, unread, unsaid, unted, unwed, widespread
We have one cheap lockpick: 'instead', there's half the universe you can write ending with 'instead'. But let's pick something more ambitious. 'unsaid' has a nice poetic ring, and we should be able to come up with something neat and moody, since we have quite enough actual hints made.
Listens to your confessions unsaid.
Nice, but we're one syllable too long. Another modern poet's friend comes in handy: Thesaurus.com. The wrong rhythm is after the caesura, and we don't want to touch the rhyming word for now, so 'confessions' gets the axe.
Who'd think, lots of results but only three 2-syllable synonyms. Still, 'stories' will be acceptable. Let's go with it.
Listens to your stories unsaid.
We can reshuffle some lines - you're rarely at such liberty with other poems, and the new arrangement both gives a nicer rhyme scheme, and composes second and third verse into one sentence:
Eight hours of death under your head.
Colorful box that's full of dreams
Listens to your stories unsaid.
Soft gifts of birds escape its seams.
There.
edit:
...and then you show it to someone and they suggest some nice corrections. Thanks, Joe Z.
Eight hours of death. Beneath your head -.
a colored box that's full of dreams.
It listens to your tales unsaid.
Soft gifts of birds escape its seams.