When I do a lone star, I put it together in quarters first (sew the points in pairs). Then I TRIM - make sure each quarter has a perfect 90 degree corner. Then you can sew pairs - and again, trim to make sure you have a perfect straight edge. And finally, sew the two halves together. If you have a 45 degree ruler, you can check the star before the first pairing. But if not, I find this works well.