So what IS a tailor tack? It's the classic way of marking fabric. Some might call it old fashioned because it requires only a plain needle and thread, but the real benefit to this low tech method is it's the best way to mark BOTH sides of the fabric. The downside to this technique is that most people sew them or cut them wrong and the threads fall out, leaving you with no mark at all. When done correctly, there is a way to make them nearly permanent!
For the sake of simplicity, let's assume that this is the only mark you need to make, so cut an ending tail the same as the first. This is the point where you would gently REMOVE the paper pattern by clipping only the paper between stitches. Again, for simplicity, I don't show this step to eliminate all steps that might keep you from understanding the basic technique. Lift the top layer of fabric.
You should now see the thread between. Keep lifting the top layer until the top of the loop is flat against the fabric, but not so much that the ends begin to slip too far away, undoing your stitch.
In 3-D, you are trying to separate the fabrics so the bulk of the thread length is in the middle with a flat stitch on top and bottom.
You can also mark your fabric with a continuous line of stitch and loop. At the end of your line, simply cut the threads that run between the loops.
Use tailor tacks to mark a seam line, darts, and symbols - especially symbols that need to be visible on the right side of the fabric. It is handy to use a different color of thread to mark your different types of marks. This helps to distinguish them from each other. Now you have a very couture method of marking!