How to make stitches invisible
You get the least visibility when you use the same colour of yarn as your fabric.
But as a beginner, it will be easier when the yarn stands out from the fabric, which does make for the stitches to be seen a bit.

I guess the blue line in the picture is the line to sew on, if so, sew on that line and not near it. The better the stitches are in one line, the less visible they will be in the end.

Also remember to keep the stitching tight, not drawing the fabric together but many smallish stitches where the yarn is flat on the fabric, so that when you look between the layers of the fabric, the fabric will be close together and there is no space for it to move.