Here we are again, finishing up for the day, as It's getting dark, and I don't sew after dark. This first photo is me pressing under the long 'slash' edge of the quarter circles, 1/2" seam allowance. I press each edge at a time, then a final press in its flat form.
Sandwiching the slashed edge between the layers of the quarter circle, I pin everything nicely ensuring that for most of this seam, the slashed edge has 1/2" seam allowance. The quarter circle's cut edge lines up with the top edge of your breeches and will be caught in the waistband later. The bottom will hang down below the slash, that's ok, it'll hang out inside the breeches front so much better once the Fall plackets are stitched in place. With the two layers of the quarter circle, and the slashed layer all sandwiched nicely, I use that felling stitch again to sew all layers at once. YES, at once! Work from the front side, not the lining, and use the fingers on your non dominant hand to feel when the needle is going through the back side. Try. If you find you don't catch the backside, that's ok, once you get to the bottom, just go back up the backside with little stitches until everything is stitched in place.
Once the inside fall's quarter circles are stitched in place, I lay everything flat and nicely on the table and then pin the bottom points of the placket in place. Then using small stitches, I sew around the point and finally using a running back stitch, across the bottom point, catching everything underneath, including the quarter circles a bit. Remember you've pinned everything flat, so once you sew, everything should be nicely in place and nailed down.
You could stop here and move on to the waistband and back sections of the breeches, but I'm a glutton for punishment ;-) and am going to make dog-ear pockets on these ones. Tomorrow. Now, it is officially too dark, and I am out of brain.