If it is just a small amount of difference between two blocks that you want to sew together, put the larger one on the bottom and allow your feed dogs to ease in the excess fabric.
As a new quilter, my biggest mistake was 'ironing' my blocks instead of 'pressing' them. 'Ironing'-moving the iron around on the block-can distort and stretch the block. 'Pressing is putting the iron on the seam for a few seconds and then removing it.
We really like pictures and they are often more informative than a thousand words.