Ina,
Are you pinning with only one pin? Try putting a pin on both sides of the seam after you've butted them up against one another. I've just discovered some two pron pins. They look like a hair pin. Really neat!
Be sure to iron your sections before you try to put them together. Big, big help! For awhile, I'd just sew the seam part, peek to see if it was okay and then sew the whole seam. Easier to rip just that little bit than the whole seam.
Bottom line it's just going to take practice to see what works best for you. :-)