You'll sew the squares into three rows, and then sew the rows together. Placement is key here!
I will typically lay out the squares in the pattern I want on my work surface and then sew them. That way you have less chance of sewing the wrong bits together because you can see how the finished block looks.
Remember that you are using a 1/4 INCH SEAM ALLOWANCE. (Look at step 12 for ways to make this easy.) :D Also, I don't pin the squares together as I sew - feel free to do it at first, but trust me, it'll only slow you down!
Starting in the top row, take the first two squares and sew them right sides together. Then, sew the last squares right sides together with the middle square. You've completed a row! Do the other two this way. Once all the rows are done, finger press the seams open.
Then, sew the rows together in the same manner, making sure that the seams line up and lay flat. The key to pretty blocks is getting all the seams sewn flat. The back should look like the last picture if you've done this correctly. :D
The pictures will help you figure this out if you're having any problems.
Once the blocks are sewn, just stack em and keep sewing until you have 48!
(A note about ironing: I don't do it at this point. I just don't. It takes up too much time. See the next step to learn about finger-pressing.)
P.S. If you're interested in learning how to add a border to your quilt blocks, check out this instructable: