accurate cutting
Accuracy starts with the cutting. Before the cutting is the optional washing and for me, mandatory starching and ironing. If you use enough steam and starch, you may avoid the washing. If your cutting is off by a thread or two, that's where some of the problems start. I make sure the ruler line is ON the fabric, not off, and I use dots of sandpaper on my rulers to hel avoid slippage. Fine thread top and bobbin also help keep those 1/4 inch seams slim. I'm amazed how much a thread can affect the seam.