Here are some tutorial options, mostly in video format on YouTube, showing how to sew or assemble fabric Christmas trees:
Simple Sewing Project for Beginners: Sew a Stacked Fabric Christmas Tree
This sewing tutorial is described as easy and great for beginners, using small pieces of fabric (scrap busting).
Link (YouTube): Simple Sewing Project for Beginners Sew a Stacked Fabric Christmas Tree
How to Make a Layered Christmas Tree Ornament | a Shabby Fabrics Tutorial
This tutorial uses fabric circles that are folded and stacked, secured with a center seam (the result looks a bit like your photo, but it’s a smaller ornament).
Link (YouTube): How to Make a Layered Christmas Tree Ornament
Note: There is also a PDF with instructions and necessary materials that accompanies the video.
Folded Fabric Christmas Tree Tutorial
Another video teaching the technique of folding fabric to create a layered effect.
Link (YouTube): Folded Fabric Christmas Tree Tutorial Master Copy
What Can You Use With Small Fabric Pieces / Easy DIY Step-by-Step Sewing Tutorial With Free Template
A fabric Christmas tree sewing tutorial that mentions a free template you can download.
Link (YouTube): Easy DIY Step By Step Sewing Tutorial With Free Template