Locate a fairly simple Celtic alphabet online or from a book store. Celtic lettering can range from basic to heavily intricate, so it is wise to start with the easier version. Stroke the lines of the lettering with your dominant index finger to become familiar with the shapes.
Use a felt tip calligraphy pen if you are new to calligraphy or a classic calligraphy pen if you already have some experience. Choose a pen or a nib with a substantial width (such as 3.5mm) which will lend well to the dramatic effect of the Celtic lettering. Set up a station in a well-lit area with a drawing board or other hard material that will prevent any texture from interfering with your writing.
Hold your calligraphy pen so that the length of the wide tip is horizontal to the scratch paper; this creates the signature Celtic style. Place your piece of scratch paper on top of your Celtic alphabet (which may be printed or in book form) and trace the letters first if you feel shaky, then practice on your own. Most of the strokes should be moving downward, to the right and diagonally. Avoid making upward strokes with the pen, and left strokes are acceptable only when they are part of a curve.
Locate an illuminated letter to try a more advanced technique. Celtic manuscripts were often adorned with an elaborately illuminated letter at the beginning of a page or section, which would then be followed by basic Celtic lettering. For example, if you wanted to write a wedding invitation, it might read: "We cordially invite you..." Locate an illuminated "W" in your Celtic alphabets and practice tracing or drawing it with pencil first.
Use a fresh sheet of parchment to create your final product of Celtic lettering. Trace or draw the illuminated letter with pencil, fill it in and gently erase the pencil marks. If it includes Celtic knots or small illustrations, you may need to use an inky ballpoint or felt tip pen (between 0.5mm and 1.2mm) in conjunction with the calligraphy pen. Use the calligraphy pen to write the rest of the text.