I use that method and here is the order that I do it:
1. Make sure the pieces are cut correctly.
2. I draw the line across each square and stack them up.
3. I lay out the rectangles and put one square on each, making sure that they all face the same way. (Sometimes I stick a pin in the side that stays - across the bias)
4. I sew all of the squaeres on the line making sure that nothing shifts during the process.
5. I trim ONLY the triangle from the square fabric - leaving the rectangle behind it intact. That creates slightly thicker geese but it seems to create a more stable block.
6. I press the triangle back. If it is a little off, it won't matter because you have the rectangle as the perfect size.
7. Repeat steps 3-6 on the other side. Make sure to orient the square correctly. (Easy to get the geese going in the wrong direction).
See if it works for you.