In preparation for our coat sew-along, I thought I'd do a "back-to-basics" type of tutorial that will help with the Lady Grey pattern: how to sew princess seams. Princess seams are only tricky because one pattern piece is more curved than the other, resulting in potential differences in length and puckering. But with proper clipping, notching, and pressing, they're a snap!
Here's what your front pieces look like; they will be sewn together to form an armhole princess seam.
Now, spread the clipped side to fit the curvy side piece. Pin along the seamline. Your clips will spread out to allow the pieces to fit together.
You might want to also clip around the waist, since this pattern is curvy there too.
You're going to notch this side, staggering your notches with your clips on the opposite seam allowance.
Update: great tip from reader Josephine.h for adapting this to the muslin stage. If you think you might need to make your muslin bigger in the bust, you'll probably want to avoid notching your muslin seam allowances so you can let them out easily. Instead, use a gathering stitch on the curvier piece to ease it into the other piece. Use the notching/clipping method above once you've got the fit right. (Of course, if you're like me and will probably need to take your muslin IN at the bust, notch away!)
Update 2: Here are a few more shots of how the pieces meet up at the shoulder, as requested. Hope this helps!