Before sewing the Thirtysomething units, check that you are stitching a scant 1/4" seam allowance. Test it by stitching three 1 ½" x 5" scraps together on the long edges. The middle piece should then measure 1" exactly. Adjust your seam allowance till it is!
Place an A triangle and C triangle right sides together, matching tips precisely. Start stitching at the broad end of the C triangle rather than at the point. This will place the smaller A triangle underneath. Don't be concerned that you will be sewing through a single layer of fabric (patch C) for an inch or so. You will notice that the C triangle points in both directions. That means sometimes you will start stitching the patch at a 90degree corner and sometimes that corner is a 60 degree angle. Remember, this is the corner that will be trimmed square after sewing the three triangles together.
The seam allowance must be pressed toward the A triangle for proper alignment of the B triangle. Hold the point straight while pressing to avoid curving the seams. Press carefully to avoid tucks, particularly at the point. Remember, do not trim the point until after the B triangle is sewn on. The point is needed to align the B triangle.
Add a B triangle, carefully matching the raw edges of all three layers at the narrow points. Start stitching at the broad end of the C triangle. The small B triangle will now be on top. Press the seam towards the B triangle. Hold the point straight while pressing.
Most likely you will want to try a few before you set up any chain piecing. After you get the idea, you will end up making 40 Thirtysomething units and get pretty good at it.
A few suggestions:
Pair all the A and C triangles right sides together, offsetting them ½". This will allow you to check that pieces are indeed RST and that you are using the correct piece. If you are not you will see the square corner of the small triangle instead of the 60degree corner.
Flip and stitch broad end in first. A lot of times in classes the gals will somehow spin the pieces so the broad end is in first with the small triangle on top and then by mistake they sew on the wrong edge of the small triangle. So, as you pick up each pair and flip them to sew with the small triangle on the bottom, check that you always stitch the long bias edge of the small triangle to the C triangle.
Use an ordinary square quilter's ruler and position it on the unit so the edges of the A and B triangles are aligned with the 3 ½ " lines of the ruler. The inner point of the C triangle should be at 3 1/4". Center the broad end of the C triangle in the opposite corner of the square. (The 1 3/8" mark on the ruler should meet the seams at the edges of the 3" unit.) Trim the C triangle and any excess fabric adjacent to it. Finally trim the points.
The Thirtysomething Square Up tool was designed to trim Thirtysomething units that finish either 3" or 4" including seam allowances. IF you are using this nifty tool, simply align the angled lines marked 3" with the seam lines of the Thirtysomething unit and trim everything extending beyond the edges. Then, rotate the Thirtysomething unit and tool to square up the unit to 3 ½", trimming the points and any excess fabric.
I've only made about a million of these things, so I think I have ironed out all the kinks. Literally, I have made over 70 quilts with these units. I occasionally hear from teachers that want to make the pieces bigger somehow to allow for the gals with wider seam allowances. They invariably change the angle and things don't line up as well then. I think this is a great opportunity to learn the importance of a scant 1/4" seam allowance. I hope you agree. Happy quilting.