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Don't worry about making your banner super large. You can always make it bigger later. Your local print shop should also be able to print the banner larger. Consider starting out with a 500 by 200-pixel banner. You can find the fill tool on the side menu. It looks like a paint bucket. You can leave the background white, or you can make it a different color. You can also choose to add a gradient. To create a gradient: right-click on the paint bucket, and select the gradient option. Choose the two colors you want; they are located at the bottom of the side menu. Click on your banner, drag down a little, and let go. You can find it on the side menu. It usually looks like a letter T. Click on it, and then click on your background where you want the text to be. You can find these options along the top of the screen. For the color, consider choosing something that contrasts with the background. If the text is not to your liking, highlight it, and change it by choosing the font options located along the top of the screen. You will want to save it as a Photoshop document first. This will keep the layers intact so that you can return to them and change them, if necessary. You will also want to save a printable version of your file. This will usually be a JPEG or PDF file. Check with your local print shop to see what sorts of files they can print. From there, you will be able to choose the paper you want it printed on. You will also be able to choose the size as well.
Open a new file in Photoshop and set the dimensions for your banner. Use the fill tool to set a background color, if desired. Select the type tool. Choose the font, color, and size of your text. Type out your birthday message. Save your file onto your computer and a USB flash drive. Take the USB flash drive with you to your local print shop to print it.