I can understand why I couldn't find any information on how to insert a small count canvas into a larger one, it is a complicated process. I could find references to how to insert small pieces, like a face or a flower, so I used the principles of this and extended it to suit my own piece.
1. I tacked the small count canvas behind the large count and attached it with a cross stitch and tacking that went through both pieces of canvas and then marked the centre point.

2. Before I cut the large count canvas (at the front) I stitched a row of tent stitch all the way around to hold the piece in place and then cut from the centre with a large X. (I'm not sure if this stitching was necessary but it made me feel more secure.)
3. Withdrew threads back to the stitched line and took these to the back of the canvas. I then wove these threads into the canvas.

The weaving in of the cut threads was an ongoing process.
This was the time consuming part.
It was all trial and error at this point and I unpicked my first line of light coloured wool and used a darker thread.

This was also my first experience with working with mono canvas which is made from a synthetic thread and is very thick an has little if any give in the thread.
5. After unpicking my original rows of woollen thread I used a COTTON 5PLY THREAD and stitched 3 rows of varying sized lines in tent stitch using Caron Watercolor thread. Then, I trimmed away the excess unravelled mesh.

I varied the width of the stitching, stitching over one and then 2 rows, around the join to stablise it even further.

I still had some spots of white mesh showing so I coloured this with a permanent ink pen.

I don't think I would bother going to this much trouble in the future unless I had a very special piece I wanted to use, but, it shows it can be done, and it was a good learning experience.