Eileen asked how closely to the seam should she clip. I clip right up to the seam without clipping through the seam. You want each half to fold nicely the direction you want it to go.
Use a short stitch -15 to the inch. That way the seams don't come apart without backtacking, and you can be confident they will hold well even when you clip right up to them or "pop" them at the ends when crossed by another seam so as to press them in a little whorl.
Use spray starch or fabric sizing to make your fabric nice and stiff-crisp before cutting - especially if you are cutting small pieces for a fairly intricate block. (Stick to fabric sizing if you might have a bug problem where you live, because bugs like to eat starch.) This gives the pieces a lot of body and helps you control them while stitching. I'd do this even if you are using "new" fabric that hasn't been prewashed.
If I am stitching something that I often have trouble turning out the right size - like a square in a square - I cut the triangles oversized and then square up the block part after stitching. You should be able to figure out the proper unfinished size of the block part, but if you can't just be sure to center your ruler over the block and leave your 1/4" seam on all sides. This probably often occurs for me when part of the block is set on the bias, like in a square in a square or this basket block or the points of this star.
None of these hints will help you sew the block together correctly, however. You are on your own for that. I freely admit that I am "spatially challenged" and do this much more often that I'd care to admit. So I still keep a "reverse sewing machine" handy at all times.