First, you have all your pattern pieces cut. I'm assuming you know how to cut them on the grain, so I won't go into that.
We'll be marking the two pieces shown below.
You want to mark these pieces like so:
On your front piece, mark right at the dot. I put a hole in the paper with my pin. I just pulled it right through. Make a hole however you wish.
Then slide the paper down just slightly to mark the center front line like this:
Now mark the dot on your placket piece.
Now we are ready to stitch the pieces together.
Take this piece (back) and lay it wrong side up on your work surface.
You want to pin your side panels to this back piece.
First pin the left and right edges. Be sure to match your notches.
Next, pin the front pieces to the side panels. Open the section you just pinned.
Take it to your machine and stitch all pieces together. This is also the time you'll want to stay stitch the crotch. Start at the dot and stitch a 1/2" seam allowance to the edge like so. Do both sides.
Now make your fell seams by clipping away one side of the seam allowance and folding the other side over it. I like for my fell seams to go toward the back, but you can do it however you like.
Open the seam allowance and cut one side.
Now press that seam allowance over the cut one. You may need slight tugging to keep excess fabric out of the way.
Then stitch all four seams down right on the edge.
Fold up the left edge of the placket 1/2" and press.
Now place the placket on the front panel right sides together. You also want to turn up the bottom of the placket by 1/2" just to expose the blue mark you made earlier (at the stay stitch line).
Stitch across the top, down the side and just to the blue dot.
On the other side, fold in the placket on the blue line you drew before and press. Also fold in the other edge 1/2" and press.
Then run a 1/2" seam allowance across the top.
Now turn the boxers so the wrong side faces up and fold down the upper part 1/2" to start the elastic application.
Start on the left and slide the elastic inside your placket about 1/4" past the center line. Pin using a large quilting pin.
Now repeat with the bottom edge then the middle. You can remove the pins once you've stitched the bottom edge.
Now stitch from that one small line you made earlier at center front to each of the lines you just made attaching the elastic.
And it should now look like this.
Right below the right front placket at the stay stitch line, clip the edges like so.
Now take the top left placket and fold it over the top right (as you are looking at it) and pin.
Put another pin at the bottom of the placket.
Turn and stitch to the edge.
Follow the instructions to stitch, and it should look like this.
And it will look like this.
And if you have questions or if I didn't explain something correctly, feel free to email me.