A gusset is usually a diamond shaped inset at the underarm, and effectively lengthens the underarm seam - ie the distance between elbow and waist. It allows a closer fitting bodice and sleeve, while still providing room for extension of the arm. Variations exist - the diamond may be comprised of two triangles, it may be shaped with a contour dart, or it may be cut in one with another garment panel. The dress I am sewing at the moment has a short kimono sleeve with a combined undersleeve and gusset:
The corners of the gusset are set into reinforced slashes in the bodice panels (those markings that look a bit like darts on the pattern), and are also set into the points where the side seams join.
It is not generally necessary to underline the sleeve gusset, but the gusset I am sewing is cut in one with the undersleeve, and as the rest of my bodice is underlined I've chosen to underline for consistency.
When I underlined I machine basted the slash line to the pivot point, to hold the layers together and mark the exact point. You can also see the notches in the seam allowance about 1cm above and below this line which mark the stitching line:
Before you cut the slash into the bodice, it needs to be reinforced. This also gives you a seam allowance to work with at the slash point. I cut some 5cm squares of silk organza on the bias, then sewed them to the right side of the bodice panel centering them over the point. Using a short stitch length, sew along the stitching lines taking a short stitch across the point:
You can use any lightweight closely woven fabric, it will not be visible in the finished garment. If you think your main fabric is likely to fray, also fuse a small piece of lightweight knit interfacing to the wrong side around the point.
Slash to the point, turn organza square to the wrong side and press:
Next I sewed the side seams of the bodice from the bottom up, stopping exactly at the crease formed by pressing in the last step:
If your gusset is a diamond shaped one, you will also sew the underarm seam of the sleeve at this stage.
The gusset has seam allowances of equal width all around it, but the seam allowances on the bodice panel vary and converge to zero at the slash point. Therefore when sewing, it is the stitching lines that must match rather than the cut edges. For this Burda pattern, each corner of the gusset is numbered - make sure you match these to the correct numbers on the bodice! Start with right sides together and the gusset lying underneath:
Beginning at the sleeve hem sew towards the first point, remembering to align the stitching lines and not the cut edges. Stitch directly alongside your previous stitching, in the crease formed by pressing:
Notice how the organza square is giving us a seam allowance towards the points, where there would otherwise be just a few threads! Stitch as close as you can to the point (within 1mm), and stop with the needle down:
Raise your presser foot and pivot the gusset so the next stitching line faces towards you, I've rolled back the bodice so you can see what I mean:
Now pivot the bodice panel to the left so that your new stitching lines become aligned:
See all that fabric behind the needle? It needs to be laid flat so you will not have corner puckers. With the presser foot up and the needle down, slip your left hand between the two layers and smooth it out - so the top and bottom layers are flat right up to the back of the needle:
Now we are ready to sew the next pass, towards the underarm point. At each turn, make sure you start and finish at the correct point on the gusset itself - ie where the stitching lines intersect. As you approach the underarm point, fold the seam allowances out of the way and stitch right up to the point, finishing with a backtack:
Halfway there! Now restart at the same point on the other side of the underarm seam - I place the needle down at the start point, rearrange the fabric to lay flat, then the third pass can be made towards the second slash point:
Again - stop with the needle down, swivel your fabric into position, flattening it to avoid puckers, and sew to the sleeve hem:
You can see I have a bit of a mismatch - I tried to get away with it, but had to unpick and fix instead as it affected the way the sleeve hem rolled - it just shows how important it is to get everything to match correctly. Some people never learn...
Check that the stitching lines on your gusset are even:
And that there are no puckers at the gusset corners:
And you're done! You can trim away the excess organza and then press the seam allowances towards the bodice. If your garment is more casual you can edgestitch the seam allowances, which provides stronger reinforcement. I don't want to edgestitch on this dress, but I am going to catch-stitch the seam allowances to the organza to help it lay flat.
Burda have some rather confusing instructions for the next step - so I'll give you my interpretation next up, along with finishing the remainder of the bodice.