After folding my fabric in half, matching my selvege edges, I place my ruler using the fold to ensure I have a straight edge. I always verify all four edges of the fabric pieces will be trimmed.
To round my corners I use this ruler manufactured by Katie Lane Quilts.
The ruler is called Corners Radial Rule and is very easy to use. You simply line the two straight sides with the sides of the piece and trim along the curved edge.
See how nicely it trims the corner? These rulers come in two sizes and each size has two sizes of corner.
A word of caution, flannel produces a lot of lint in the bobbin case so be sure to clean your bobbin case out after stitching your seams together. I also add a drop of sewing machine oil on the race of the bobbin case. I do not recommend using canned air because it can add moisture onto your machine parts causing damage. Also, you don't want to risk forcing small pieces of lint or thread into areas you can't see or reach resulting in stitching problems.
After I've stitched my seams, I clip each corner so the seam will lie flat after I've turned it to the right side.