Most "Keep Calm" posters have the words centered. There are usually five to six lines. The first  four words are on their own line. The crown is at the very top, also centered. The original poster was red, but you can use any color your want. Some colors might be more suitable for certain posters than others. For example, pink might be suitable for a poster that says "Keep Calm and Go Shopping," while green might work better for a poster that says "Keep Calm and Garden." Most "Keep Calm" posters are a solid color, however, many of the newer posters have a patterned background. Keep in mind, however, that a background could make the words difficult to read. If that happens, try using a paler or a faded background instead. Here are some ideas to get you started:  If you are making a patriotic poster, consider using a flag as a background. If you are Star Wars, Star Trek, or Doctor Who themed poster, try finding a black background with stars on it. Most of the font used is a sans serif font, such as Ariel. The letters are straight lines, and don't have any hooks at the ends. However, some people like to use a fancier font for the last word or two. Make sure that the font is all upper-case or capital letters. Most of the lettering on Keep Calm posters is white, but if you used a really pale background, or yellow, you might want to make the letters darker. You can also make the last word a different color. The traditional poster used a crown, but if you are making your own poster with your own phrase, you can use a different image. Whatever image you do use, however, make sure that it is a silhouette, and the same color as the font. Here's some ideas to get you started:  If you are making a super hero themed poster, consider using the super hero's logo instead. This is the exception to the same-color/silhouette rule. If you are making a coffee themed posted, try using a coffee cup or a coffee mug. If you are making a poster based off of a princess movie, then use the princess's crown instead.

Summary:
Know the layout for the text and crown. Use a solid color for the background if you want a traditional poster. Consider using a pattern or an image for the background. Choose your font. Decide on a color for the font. Know that you don't have to use a crown.