In your graphics editing program, open a new document. Be sure to specify the size based on the size you want printed. Posters can be anywhere from 11" x 14" up to quite a few feet high, with some more common sizes being 16" x 20", 20" x 30", and 24" x 36". Signs and banners can be similarly sized, or much larger. Determine the size of your document based on the sizes your printer offers, whether you print it on your own printer, or send it to an external print shop. If your program asks for image resolution, set it to 300 ppi (pixels per inch).
It's important to now set up margins, including a "safe zone." This tells you how close to the edge of the document you can go without concern. Past this area, and you run the chance of the text or graphics being trimmed off at the printer. You also need to determine the bleed area. This refers to the space beyond the document you will need to fill to ensure that area is filled with your artwork after the document is trimmed by the printer. This distance is often 1/8th of an inch beyond the edge of the document, but ask your printer for the bleed to be sure.
It is also useful to set up guides. These do not print, but appear on screen to help you center and align the elements of your design.
Now you can begin to actually create your poster, sign or banner. Consider a background for the poster, sign, or banner. It can be a color, gradient, pattern, or photographic. Just pull the photo image into your graphics editor, and position where you wish. If using a photo, make sure it is at least 200 ppi at the size it will be printed. For example, if the photo will be printed at 8 " x 10" in a poster, it should not need to be "upsized" to 8" x 10". In very large material, like a large banner, it is possible to go larger. The quality of a photographic image, if upsized a great deal, will not look great close up, but should look fine from a distance.
Open your program's text tool, and experiment with different type fonts until you find something clear and readable. Don't forget to include the main point of your poster, sign or banner. What is its purpose? Do you want to call people to action, direct people somewhere, or just raise awareness of a cause or deadline? Create a headline appropriate to the purpose, and that matches the message your text conveys. Try not to use more than three different fonts in one single document.
Keeping things simple is not always easy, but is the best way to make your message stand out. If you include extra decorative elements, like splashes, stars, or other attention-getting graphics, do so only to point out something the reader needs to know, like dates, contact information , etc. It is also sometimes a useful to draw a border or box around information you would like to separate and call attention to, but avoid too many boxes.
For a poster or sign, it is often fine to save your file as a high quality jpeg image to send to a printer. For banners or very large documents, this will often create too large of a file. In this case, you printer may request an .epf of .pdf file. These files are often preferred because they can be edited and resized by the printer if need be. If the file is too large to email, your printer should offer an ftp upload option.