Hi, Marilyn -
Here's a tutorial that shows up close the working of a center thread:http://www.needlenthread.com/2009/02/dr ... -loop.html
It doesn't show close up how to secure that thread, but it really depends on how you treat the ends of the area where the threads are removed. In the photo in the tutorial, you can see that the edge of the area is treated with satin stitch. If this is how you treated the edge areas where you withdrew the threads, then you'd secure your thread underneath the back of the satin stitches. If you finished the ends without the satin stitches (by re-weaving the withdrawn threads back into the fabric), then you'd have to use very discreet tiny stab stitches to secure the thread.
If you're working through an area like the one pictured in the link I posted up above, you need to estimate the length of thread according to the length of the area you're working across, taking into consideration the thread used to make the stitches, as well as enough to anchor your thread at the beginning and end.
Hope that helps!