Back basting is when you trace the block pattern on the back of your background fabric (using a pencil or erasable pen or whatever) and then attach the front fabric to the back fabric with a basting stitch done from the back along the exact line that you traced. After that you applique via needleturn from the front, cutting the fabric and thread just an inch or two ahead of your needle. It's more straightforward than it sounds. It's also pretty accurate. Very accurate, if you're good at it.