Write out a few lines, counting to make sure each consists of 10 syllables. The English language naturally follows an iambic pattern, so you may find that many of your lines naturally fall into the iambic rhythm.
Draw a line between every two syllables in your lines, such that each line consists of five two-syllable feet. Mark the stressed syllables with a diagonal line above the syllable to help you visualize the iambic rhythm. A stressed syllable is one that is emphasized, such as the second syllable in the word "iambic." Say each metric foot aloud, and pay close attention to which syllables are given a natural emphasis. Generally, all articles and prepositions are unstressed, whereas polysyllabic words contain at least one stressed syllable.
Revise your lines such that each metric foot consists of an unstressed and stressed syllable, respectively. You may have to reword the lines to work in words that follow the iambic rhythm. Technically, only unrhymed lines consisting of 10 iambic syllables are considered blank verse lines. However, most blank verse literature occasionally breaks from iambic pentameter to add a varied rhythm.
Continue to write and mark lines of unrhymed iambic pentameter until you develop a natural rhythm for iambic pentameter. After practice and repetition, you will find it much easier to hear the iambic rhythm in your head as you write, and your lines will more naturally come out in the pentameter format. It is also helpful to read other work written in iambic pentameter aloud to help you pick up the rhythm, such as the blank verse poetry of William Shakespeare and William Wordsworth.