As already stated you do not need to sew an X is you are not using batting.
Also if you want to sew an X on the rest of your sandwiches, you do not need to start in the corners. You will be having a big seam allowance (up to an inch/2cm) and you do not need to stitch into the seam allowance.
Your flannel layer will show when you do the snips, (unless you cut it smaller than the outer layers (then you would need an X). The flannel will add to the ragged edges and it will look good.
When you get to the snipping stage your snips do not all have to be the same distance apart. The closer the more ragged the result.