Writing poems usually means letting the lines flow onto a page or into a computer then going back later to revise. At that point, it usually helps to read your work aloud, listen to the poem, hear the form that seems to suit it best, then recast the words or lines until you have lively line breaks in a free verse poem or the formal form found in a heavily structured pattern such as a traditional sonnet, limerick, or villanelle.
You can revise or rework a poem to get haiku and other types of syllabic poetry too. More likely though, a poem that’s based on the number of syllables per line will start, not as you tap your foot or count feet into lines of regular meter, but as you count each syllable on your fingertips.
Take haiku, for example. To write those three lines of traditional Asian verse, you need 5 syllables on the first line, 7 on the second, and 5 on the third. Traditionally, you need to refer to some season of the year, too, touching your pen lightly to the scene you sketch, quickly and exquisitely, with your words.
Knowing the background of any type of poetry can help you to write or revise well. For instance, haiku comes from ancient cultures that developed the form as a means of entertainment at social events, so a traditionally written haiku often has the levity found in party talk.
Assuming you do not readily read Japanese or Chinese poems in their original languages, your introduction to haiku will probably come through one of the excellent translations found, many centuries later, in most bookstores today. This means, however, that poems translated from one language to another will vary in the original syllabic count. So an English version of an ancient poem, say, by Basho might have 2/4/2 syllables on their respective lines, rather than the 5/7/5 syllables the poet initially used.
Over the years, poets who write in English have varied the count of syllables and the number of lines, omitted seasonal references, handled hot and heavy subjects, and called it haiku, when they really have their own innovative verse set as a short syllabic poem. What you do is up to you, of course, and also the editors of journals or e-zines where you plan to send your haiku in hopes of getting published. Personally, though, I prefer the original 5/7/5 form because of the appealing pattern but also because, when I write haiku regularly, the words and thoughts just seem to fall into that mathematic formula or sound.
Actually, the same can be said for writing in traditional English forms that rely on, say, iambic pentameter. After a while, the lines seem to slip into your thoughts, already shaped into your chosen meter.
You can find out more about formal and informal poetic possibilities in the e-book version of the poetry home study course I wrote and used for years in working with other poets but also in the poetry dictionary e-book shown on this page too.
(c) 2011, Mary Harwell Sayler