When I sew the side triangles to the blocks I sew one side and then the other. I pinch the centre of the block and the centre of the triangle and match them up and pin securely. Then I move outwards -- carefully -- and continue to pin. These little ones I only used 3 straight pins on them.
Once both sides are done and I do chain piecing on them to move things along a bit faster, I take them to my ironing board. Well, not quite an ironing board....................more like an ironing ?????? --block? I have this almost antique iron that has taken more than one licking and works like a charm.
It is a GE and was given to us for a wedding gift 45 years ago this September. No steam, just good solid heat. I press the seams to set the stitches and then open them up giving them a really good press.
TIP: If the two points of the triangles meet in the centre of the block your triangles are centred.