Starting off, I think the hardest thing is picking your fabric. I can sit for a very long time, mentally composing a diaper. When it comes time to go through my fabric, I'm honestly overwhelmed by all the choices I have.
Once you've picked your outer and inner fabrics, decide what colors you're going to use for the ruffles. For this tutorial, I'm using 2 inch strips of PUL in solid colors that match (or come pretty close to matching) the printed portion of my outer material. I like to cut the strips twice as long as I need them to allow for maximum ruffling. In this case, I just used some strips that I already had (the light pink), and then cut 2 other strips for a total of 3.
For the next row, I put it so that it's just covering the zig-zag from the row below it.
Before I sew the inner, I roll in and secure the edges of the bottom row to make sure I don't accidentally catch them in the seam allowance.
After this point, continue sewing your diaper as normal...inner to outer, right sides together, etc etc. And through the magic of the internet, here's the finished diaper...