Embroidering letters is a quick and simple way to add a personal touch to an item, or make a big statement about something you want to say, be it a cross stitch sampler or something for your next protest demonstration.
Things to consider
What sort of fabric are you using?
You need to distinguish between normal fabric and evenweave fabric. Evenweave has threads in both directions of the same thickness, so you can count the threads up and down to make a square and is mostly used for cross stitch. Any fabric that doesn't have evenweave is likely to have threads of different thickness, so counting the threads would give uneven results. You can plan letters for evenweave fabric in squares and there would probably be a suitable alphabet in any cross stitch book. Normal fabric can be much more liberating as there are no squares to follow and you can let your imagination run wild.
What do you want to say?
Adding your initials, called a monograph, to an existing item like a bag is a common task and is quicker and easier than cross stitching a witty saying on a sampler. Try working out how the letters will look using a scrap of paper or even graph paper. A computer can help too, with lots of fonts to give you ideas for different types of letter styles, or try looking at a calligraphy book. You can experiment with layouts and work out the best fit for the space you have available. Don't be afraid to be bold.
Getting the design on the fabric
For cross stitch lettering, there is no need to transfer the design because you can follow the letter chart by counting the squares and stitching where indicated. For other lettering, you need to transfer your design. If you're feeling bold, you can copy your design freehand using a very light touch with a normal pencil or a washable fabric pen (available from quilt shops and commonly used for marking quilt designs). Also available from sewing shops is a transfer pencil where you draw your design then iron it onto the fabric, but you must make sure to reverse your lettering so you don't end up with mirror writing. Dressmakers carbon paper acts in a similar way. Depending on your design, you could cut out your letters and draw round them. Look at an embroidery book or magazine to see other ways to transfer designs but remember the stitches have to cover all the markings.
Stitching the letters
If you are stitching the letters simply and clearly, try a basic line-type stitch, like back stitch, stem stitch or chain stitch (details available in any basic stitch book). Using a relatively thick thread for the fabric might help the letters stand out. Alternatively you could add a second thread by "whipping" the first stitch, that is sewing through the first line of stitches on the surface of the stitch with a second decorative thread. Again, details would be in a stitch dictionary. For bigger letters, you could outline each letter, then add a filling stitch for the body of the letter.
Don't overlook your sewing machine for embroidery. Many basic machines have the ability to do free machine embroidery, which is the equivalent of using the machine needle as a pen. This is great for sewing letters and text of all kinds, but it does take a bit of practice so look in your manual to see how to do this and check out sewing books for the technique. Many computerized sewing machines have built in stitch alphabets too, although they can be rather small. Once you have the sewing bug, you could try appliqu: cutting out letter shapes from different fabric and sewing them down by hand or machine which can make a really big impact.
Why not try all these methods, and see which suits your needs? Before long, you can have all your personal items covered in embroidered letters, and then move on to items for your family and friends for great individual gifts.