When you're couching (not the potato couching...weak laugh) you are usually using a heavier thread that shouldn't go through the fabric because it will start to look frayed. Poke a hole big enough to get the thread through from the top at either end of the line the thread will be on. Use a matching colour of thread or contrasting if you like and stitch close to the thread to be couched down - up close then down close. Don't pull tight as you don't want to see the couching in thread on the couched thread. Probably clear as mud but hope this helps to get you started. Don't hesitate to ask more questions.

If you need visual aids try YouTube. Many good videos.(Because my daughter lives so far away and I don’t explain things well it’s where she learned to do French knots.)