Now that you've had enough time to admire your gorgeous welt pockets, we can move on with the construction.
At some point, you're going to want to reinforce your hems. There are a variety of ways to do this. For this blazer, I used knit fusible interfacing strips pressed centered on the hem line.
I chose to do it this way because the finished feel I wanted for the jacket was lightweight with just a bit of body. To keep the bulk down, I pressed on the interfacing keeping it out of the seam allowances. It's easier to do this while the pieces are flat - i.e. before sewing up the side seams.
Of course, you could also do this right after cutting out your fabric, before sewing up any seams. Depending on your fabric and the finished product you're going for, it may not matter much if you have interfacing in your seam allowances.
You could also opt to use wigan (my favorite!). I didn't use it on this jacket because the wigan needs to be catch-stitched to the outer fabric. Since my fabric is smooth and tightly woven and I didn't underline it, those stitches would show on the outside. So I used fusible.
If you choose to use wigan, place it so that 2/3 of the tape is above the hem line and 1/3 is below. Again, you can choose whether to add the wigan to the pieces before sewing any seams, or wait until the entire body is constructed. I also find it helpful to baste along the hem line - it makes it easier to turn the hem later. Here are a couple examples from previous jackets:
The next step is to sew up the side seams and press them open. At this point here's what you've got:
Now pin the shoulder seams together, matching up the notches. In general, the front shoulder seam will be a bit narrower than the back, so ease the front into the back. This will be easy to do because these edges are cut on the bias.
Take a piece of your twill tape (it should be pre-washed and pressed) and line it up with your front shoulder seam line. Make a mark at the end. Then make a second mark 1/4" inside that first mark.
Stretch the tape along the seam line to that inner mark and pin in place. Adding twill tape to the shoulder seam helps ease the seam in further and stabilize it.
Sew the seam directly on top of the tape, then cut away the excess.
Do not trim these seam allowances. Press the seam open over a ham rather than the tailor's board. Because the longer back edge has been eased into the shorter front, the seam should curve toward the front ever so slightly.
That's all there is to it! Once I got to this point, I placed my jacket and under collar next to each other to admire my work:
It's really starting to look like a blazer now!