This first picture shows how I hold the piece in the middle at the seam.
Can you see how the seams lay? The seam line on the thumb side lines up perfectly with the raw edges on the bottom piece. When that lines up then the raw edge on the thumb side will line up with the seam line on the bottom piece. Drop a pin in there.There will be a 1/4" hang over on each side. Drop a pin in there.
I press the seam open just to cut down on the bulk. You can see how everything lined up and made a good diamond.I had so much fun and they go together so fast I had a BIG pile of them before long.
I hope I was able to explain what I did and I hope you try diamonds.