Take the poster off the wall; remove it from its frame, if necessary.
Set the iron to its lowest setting and allow it to heat up.
Lay the poster out on a large, flat surface such as a table. Make sure the poster is face down, with the blank side facing up.
Cover the poster with the sheet or towel and lightly spray the material with water. Your fabric should be damp, not wet.
Run the iron over one small corner of the fabric. Do this until the fabric is dry and then stop. Check the poster underneath for any heat damage. This should not happen if you iron is set low enough.
Iron the entire sheet or towel and poster, letting the gentle warmth dry the fabric and flatten the poster. Check the poster and repeat the process if there are wrinkles left.
Reframe and rehang your poster.