Read blank verse poems to get familiar with the rhythm. Many of Shakespeare's plays are written in blank verse, as are famous poems such as Robert Browning's "The Bishop Orders His Tomb at St. Praxed's Church" and Robert Frost's "Birches." Read these poems aloud to get a sense of how blank verse sounds before you attempt your own poem.
Write your poem in roughly ten syllable lines -- at this point, do not worry about meter. This way, you can get your ideas on paper first, then conform them to the proper meter in revision.
Take each line of your poem and revise so that your lines are primarily iambic meter. Remember that iambic meter means that the first syllable is unaccented, the second accented. Aim for five iambs in each line.
Review the length of each line and make sure that your lines are still around ten syllables. In arranging your lines into iambs, you might have expanded some of your lines and added extra syllables. Try to cut any extra syllables and rearrange the lines to fix any metrical issues.
Read your completed work aloud and listen for regularity in your meter. This is an essential step after you have examined each line individually. Listen to make sure that your meter is relatively smooth.