I highly recommend clicking the "Watch on Youtube" link in the video as this low quality won't allow you to see what I'm talking about. Video is available here: Watch video.
When we sew quilt blocks, we usually do not sew with a dual feed presser foot, which causes a slight stretch/tension in the fabric on top compared to the fabric being fed at the bottom by the machines feeders; which is why the top piece usually comes out longer than the bottom one, despite being totally equal in length when the seam was started. This causes the fabric, when the seam is sewn to have a slight "waviness" to it, as the fabric has been stretched slightly. This can be made worse from improper thread tension. Adjusting your presser foot pressure (usually reducing it) so your seams come out as flat as possible, while still retaining enough pressure to make a nice seam will help this. However, a slight waviness is difficult to get rid of. Starching your fabrics with spray starch before you cut it is a great way to reduce this issue too. However for baby quilts such as this one, where you don't wish to spray the fabric with a bunch of chemicals is not desirable. This is where setting your seams come into play.
Setting your seams is something the old school quilters amongst us will say is such a standard thing to do that it doesn't warrant a post, but I've found a lot of new quilters don't know to do this, and it makes such a difference!
Set your seams:
Start with your block in the same position as when you lifted it from your machine, don't open up the fabrics, just lift it from the bed to the pressing table. Here you can see the little waves in the fabric.
Put your iron on the cotton setting. With or without steam is a personal preference, have you pre-shrunk your fabrics properly with a hot iron with steam, you can use steam here to without danger to distort your block. Set down your iron on the block and give it a good press. Here you can see how flat it becomes, the waviness is gone!
And enjoy your perfectly flat, beautiful block!
In the video there's a comparison with a block with the seams not set. Leave your blocks to cool flat on the ironing board, preferably even under a heavy book or a quilt ruler to let the fibers set in place flat before moving them.
Hopefully this will help you to get your blocks properly flat and beautiful!