Brainstorm ideas for the subject of your poem. It can be direct or woven within the framework of your poem. The basis of a didactic poem is to provide knowledge or instruction to educate the mind and improve morals.
Keep in mind that while a didactic poem is instructional, it isn't necessary that the instruction be for a specific task, such as how to ski. It can be more general and metaphorical in topic, such as how to love.
Examine other didactic poems to gain a better understanding of how the didactic poetic form works. Read Alexander Pope's "Essay on Criticism," which quite literally teaches you how to perform literary criticism, or Robert Frost's "Mending Wall," which more subtly tells the reader how to build a relationship with your neighbor.
Consider what style you would like to use for your poem. Among the possibilities are free verse, rhyming, or even a specific form. The term didactic focuses more on the topic of the poem rather than the style, so you can write a didactic poem using another form of poetry, such as a villanelle or sonnet.