Write down some ideas about your subject. Use this brainstorming technique to narrow down what you would like to say about your mask. For example, you might want to establish whether you want to write literally about a mask or simply write a "mask" style poem, where the subject of the poem is also the speaker.
Write down additional ideas of what you would like your subject to do in the poem and the ideas you would like to get across. For example, if you want to speak in the voice of the mask, you might want to convey what it feels like for the mask to be peered through when it is worn and what it feels like for the mask to be taken off and thrown into a drawer, where it is forgotten.
Write down ideas of key words or phrases you would like to use in your poem that convey the ideas you want to get across. For example, you might write down such phrases as "I shape what is seen," to describe what the mask feels like when it is worn, and "I am left in a cold dark place where I can see nothing," to describe what it feels like when it is put away.
Write down draft lines of your poem. Begin to shape those lines into the format that you would ultimately like your poem to contain. Say the lines aloud as you write them to get a sense of their flow. Rearrange the words and phrases as necessary until you are satisfied with how they fit together.
Finalize your poem by ensuring it meets the format you require, if necessary, such as the syllable requirement for a haiku or the rhyming requirement for a couplet. If it is a free verse poem with no particular format, read through the poem one last time to ensure it sounds exactly as you want it to and tells the story about the mask that you want told.