Here are the pieces from the first part, lined up at the sewing machine, ready to sew. Take the pieced triangle on the right, and place it, right sides together, onto the piece on the left.
The point of the background triangle lines up with the corner of the rectangle. Begin to stitch the seam, using a scant 1/4" seam allowance. Stop and line up the rest of the block. If it doesn't fit exactly, remember that one edge of the focal fabric is bias and will stretch or ease a tiny bit. Both seam allowances are facing towards you, so they stay in place while stitching. (If you have a straight stitch plate for your machine, this would be a good time to use it. Points tend to be pulled into the zig-zag slot, especially on machines with a wider stitch area.)
Now is the time to carefully trim the overhanging triangle of background fabric, using scissors or your rotary cutter and ruler. Press the seam flat, with a hot, dry iron, then press the seam open.
At this time, you should trim all the "dog ears" (overhanging seam allowances) except for the two on the focal fabric.
Repeat with the other 3 parts of the block, then lay them out in the correct orientation, with all the focal fabric pointing to the centre of the block.
Now flip the pieces on the right side onto the pieces on the left side (right sides together). Place them beside your sewing machine, and repeat with the remaining blocks. Take care not to mix up the pieces - seems obvious, but........