You've gotten lots of good advice above. "nesting" seams means put the blocks together so that the seam allowance of one block go in the opposite direction from the other block. Pinning does help. A lot of us "square up" our blocks each time. Press the block really well. I suggest you use a dry iron as a steam iron will possibly stretch your fabric. Remember to press not iron. then, once the block is well pressed, measure it to see if it's the right size. If it's overly big you can cut it back to the correct size. If it's too small, you need to check your seam allowance measurement. I think you are doing really well. One way to immediately see a problem is if the "dog ears" that show up in your first picture are evenly sized. Yours are - a good sign. Keep going!