There's a couple of things you can do. One, you can "nest" your seams. When you sew a seam, flip it over and press the allowances in one direction. For the next block, press the allowances in the other direction. Then, when you sew these two blocks together, the seams will sort of nest or lock. If my explanation isn't making sense, you can google "how to nest seams" and you should be able to find videos or illustrated tutorials.
Another thing you can do is pin. I like to do this especially with blocks where I've pressed the seams open. Stick a pin in the seam, between the two patches, a quarter inch in from the raw edge. Lay the block on top of the next block, stick the same pin in
that block a quarter inch in from the raw edge. Dab a dot of Elmer's washable glue in the seam allowance, touch a hot iron to dry it, remove pin, and sew. Again, if this isn't making sense, google "How to fix mismatched seams" and you should be able to find illustrations.
hth
