Decide on your poem's style. When choosing the poem that you want to base your pastiche on, think about poems that are nearest to your own "voice" or perhaps poems that you wish you had written yourself. In your pastiche, you must try to emulate and reproduce the skill of any chosen poem. A pastiche should not simply aim to copy the unique rhythm and rhyme pattern of the original, but should also attempt to capture the tone.
Define the theme. Think about the poem, its theme and its setting. Decide on what you are writing about. If the original is about being at sea, you can make yours about being in space; if it is set in the past, set yours in the future. The poem changes, but the writing remains the same.
Take the original a step further. Writing a pastiche is a learning experience more than anything. In writing, you are forced to face and solve any writing problems that may have occurred during the writing of the original. The objective of writing a pastiche is not to simply reproduce a feeble version of an original piece. A pastiche is written to learn from the writer and hone your writing skills.