First off, I will note that in these pictures, I used 14-count Aida (which I stitched over two) and two lengths of different-colored floss. Each length corresponds to one strand of floss on your workpiece (assuming, of course, that you are working with two strands; there is a note about using more at the end of this tutorial). These strands may or may not be the same color; if you are using one strand each of two different colors, it produces and effect called "blending" or "tweeding".
1. Gather your materials.
You'll need your workpiece (stretched in whatever frame you prefer, unless you're an in-hand stitcher), your threads, and your needle. This is pretty self-explanatory, but I've included a picture anyway. (P.S. - don't mind my messy desk!)
2. Bring your needle up through the fabric.
Begin your first stitch by coming up through the fabric. The lower leg of my cross stitches go from lower left to upper right (/), so I will be heading to the upper right next.
3. Stretch the floss across the fabric.
You now want to stretch your floss across the fabric, in the direction of your stitch. You want to stretch farther than the hole that your needle goes down. Now, smooth the stretched threads so they are not twisted around each other, putting one thread on each side of the hole. It is exaggerated here, so you get the idea.
Note that you are not really "stretching" (i.e., putting tension) on your thread. You are really just taking it across the fabric.
4. Put your needle down through the fabric.
Place your needle between the two stretched strands, and push it down through the fabric. You will pull the threads through straight and they will remain parallel and untwisted. I find that longer lengths can still twist at this stage, so you may have to use your non-needle hand to keep the threads flat to the fabric as you pull. You can also use a laying tool to keep the threads flat, but I will not go into that here.
This is also a good example of what stitching "over two" means. You bring your needle up, go right two holes and up two holes, and put your needle down (alternately, you can count diagonally two holes, but I find this tricky on linen where not all the threads are of a uniform thickness). The "over two" method is typically used on higher-count fabrics: 28-count stitched "over two" would give the same stitch size as 14-count stitched "over one".
5. Complete the bottom legs, and start the tops.
Once you've completed the bottom legs of your stitching, you can start the top legs (unless you stitch in the English style, which I believe completes one whole stitch before moving to the next). This is where the railroading is really important, as your top legs are more visible than your bottom legs.
6. Complete stitching.
Here is the row of five stitches, completed. As you can see, I have been very careful to consistently keep the same color on the same side of the stitch. Here, the white thread is on the "bottom" (or "left") and the black thread is on the "top" (or "right"). It is not necessary to do this. My personal experience has shown that some areas look better with this consistency, and some areas look better with letting the threads randomly lie (though they are still railroaded), as show in the blue/black section of the next picture:
For those who still can't see what difference railroading makes, I direct you to the next picture. If you look at the stitch indicated by the red arrow, you will see that the strands are crossed: both strands come up through the fabric, but the blue crosses over the black before they go back down.
A Note About Multiple Strands
It is still possible to use the railroading technique when there are more than two strands of floss in your needle. You will still come up, smooth your threads, and go back down, though it can be tricky with 4 or more threads. A laying tool (or a spare needle) may be needed to smooth and hold the threads while you take the downward stitch. (If you need pictures of this, ask and I'll see what I can do).