In writing mud I generally write in messy prose and verse for a while (sometimes pages of it) -- keeping the idea, phrase, or mood in mind while pumping out words and sentences in an abstract, cluttered way. From this recorded stream of consciousness I may recognize patterns of sound or rhythm, length of line, rhyme schemes, or repetitions. If these patterns are close enough to the patterns found in a remembered classical form (and if the form is proper for the poem), I "shape" the mud into the form. This is accomplished by moving words and phrases around, finding synonyms to fit a meter or rhyme, and throwing out or adding words and lines as necessary. Sometimes this is quite a chore, other times the mud just falls into place. On a rare occasion my mud is so close to a classic form that very few revisions have to be made. An example is Writer's Haze, where I had written these lines as mud:
 

... and so on. While reviewing this stream of consciousness writing I recognized the meter as iambic pentameter right away (easy enough to do in the English language), and then was startled to see that the rhyme scheme fit a Shakespearean sonnet nicely. Very few changes were made to shape the (very structured) mud into the beginnings of a sonnet.
 

More was done later to spice up the poem, but it was basically finished once it was in sonnet form. I hope that as I write more poetry and become more experienced the mud will more readily fall into place as it did for Writer's Haze, but for now I often have to spend long amounts of time and patience shaping the mud into a form. Poems rarely write themselves.

If I recognize a strong pattern in the mud, and that pattern doesn't come close to any known form, I usually accept the pattern as an invented form, analyze it, and try to shape it further into my new form. These invented forms can be just as intricate as any classical form, and just as challenging.

If it is a technical structure which is inspiring me to write, I don't always begin with a mud-writing session. I first research the form and attempt to find and deconstruct other authors' works written in the same structure. Then I play with words and phrases, pushing them into the template that the form offers, looking for what seems to work and trying for some sort of prosaic cohesion-that is, I try to get the words to scan easily, so that if the lines were written together (without being broken into stanzas) the poem would almost read as prose. 

I also search for the moods and limits of the form; for instance I have found that it is easier to write humor in heavily metered, rhymed couplets than, say, blank verse. 

I do not always try for a decent poem when I am playing with a form -- I'm writing for exercise (and to just experience the form), and I don't care (at first) what words are pushed into the template. If a poem I am happy with comes out of the exercise, so much the better. I sometimes finish playing and throw all my form exercises away. It all helps later, when I have an idea that I know can fit the learned form, or when I recognize the form in my mud.

My tactics when writing a first draft of a poem are similar to those used when creating a piece of ceramic art. I either start with words (the clay) and shape them into a form, or I select a preset form (the mold) and push the words into it. After one of these methods produces a first draft of a poem, I begin the next stage-revising the poem.