B..
Start at the top, don't smear your marker by putting your arm on your work. Make a light pencil line so your letters are roughly straight.
Most of all, make a rough draft on paper before you do it on the poster board. It make you think about what's going where and what to cut.
Spell things on paper, first.
Fill up the poster. Not 19 little things, 1-3 BIG things. That would be good to practice on that paper. Most of the time they draw too small.
Write the concept steps on the board. Along with a drawing of a poster or two. They could see you drawing BIG.
Name the cool colors- blue, green, purple Warm colors- red, yellow, orange
Use two of one (warm or cool ) with a touch of the opposite. Makes things pop!
After they start, try not to talk. Talking is a left brain thing and your right brain won't work as well for creativity.
Youi have some big concepts here. Your best students will appreciate the lesson and use it well. Some will be overwhelmed. They are not visual people and the concept of counting letters and stuff will leave them in the dust. Talk about everybody has strengths and differences.