A favorite site with helpful info here with other video tutorials.
https://www.sailrite.com/french-seam...ction-tutorial
What type of thread are you using for sewing? I have ordered and used their heavy threads and they held up well with my projects for our boat.
https://www.sailrite.com/Notions/Threads/Indoor-Thread
Last one is a thread guide.
https://www.sailrite.com/Selecting-t...ht-Thread-Size
Good luck with your project!