First, start off by trimming down the HSTs after you choose which method you'd like to sew them together. I trimmed mine down to 1 1/2 inches each. Excuse my first two pictures. For some reason, the blog wants to turn them. You get the gist of it. Make sure you have your 45 degree angle lined up with the center seam, otherwise they won't end up correct.
Then sew them into rows.
Then sew the rows together. It's totally your preference on how to press the seams. I press towards the squares and then I press to the inside of the completed 3 1/2 inch square. You may choose to press them open so that the block lays flatter. After you give it a final trim (again utilize that 45 degree angle mark on your ruler and go corner to corner), you'll have a 3 1/2 inch unfinished square. I have 92 more HSTs to put together, so I should have enough to make another baby quilt after I add borders or sashings! Enjoy!