My first tutorial! Hopefully this makes enough sense...
This tutorial is for a box pleat crib skirt or otherwise know as a tailored crib skirt I believe. This picture and fabric does not show the box pleat very well. There is just one box pleat in the center of each side of the crib. There are no pleats on the ends.
Step ONE: Cut fabric.
You will need:
2 lengths of fabric that are 62 1/2" x 15 1/2" for the sides of the crib and
2 lengths of fabric that are 29" x 15 1/2" for the ends of the crib
For a crib skirt with a stripe at the bottom in accent color (for example see my valences) you will need:
2 lengths of main fabric that are 62 1/2" x 11"
2 lengths of accent color fabric that are 62 1/2" x 5 1/2"
2 lengths of main fabric that are 29" x 11"
2 lengths of accent color fabric that are 29" x 5 1/2"
Step One B (for crib skirt with a stripe on bottom only): Match up each piece of main fabric with an accent color fabric piece of the same length. Sew main fabric piece to accent color piece with right sides together 1/2" from edge (or surge to keep from future fraying). Do this with all four pieces of main fabric so that you have four lengths of fabric with the main color on top and a stripe of accent fabric on the bottom. Continue on to Step Two.
Step Two: Hem fabric ends.
Fold the narrow ends of each piece of fabric over 1/4", then fold the end over 1/4" again so that the raw fabric edge is on the inside. I hate to pin, so I fold as I stitch which is why I do a 1/4" hem on the ends. It is much easier to fold a 1/4" hem as I sew than a 1/2" or 1" hem. If you are not comfortable with folding the hem as you sew, then pin.
Step Three: Hem bottom edge.
Here you will want a 1/2" hem. Fold the bottom edge of fabric over 1/2" then fold it over 1/2" again so that the raw edge is on the inside. Pin. Sew. Of course you can always do as I do and cheat. I love
The Nester's saying that "It doesn't have to be perfect to be beautiful." So when I do things for my own home I'll take whatever shortcuts I can as long as it still looks great. So I "cheated" on this by just doing as I did on the end hems. Fold and sew a 1/4" hem instead of folding, pining, and sewing a 1/2" hem. It just makes the skirt 1/4" longer.
Step Four: Create the box pleat.
This is the fun part! Not because it's difficult, but it's a box pleat skirt so it's fun to finally see the pleat. Or maybe I'm just easily amused... Anyway I've drawn a diagram to help you see this. (And besides, I goofed up and didn't allow enough fabric to make the size pleat that I wanted to so I ended up with only 1/2" where I should have had 2". Follow the diagram and your's will be much better than mine!)
Find the exact middle of both of the long lengths of fabric and mark with a pin or marking device (if you stay within 1/2" of the edge you should be safe to mark with a pen or pencil as it will be in the seam allowance and not show in your final masterpiece). (Note that there will only be pleats on the sides of the crib skirt and not on the ends.) Then measure 4" on either side of the middle and mark. Pinch fabric at the far left mark and bring it in to the middle mark. Pin. Repeat with the right side.
Your fabric should be folded as in the diagram above only tighter and pinned with raw edges together of course instead of loose. In other words the point at which the two sides should meet is on the line right under the 4. (Does this make sense or is it confusing. I seem to have that effect on people... Confusing them I mean...)
Next stitch the pleat together 1/4" from the edge (only 1/4" so that your seam will not be visible in your final masterpiece). This will hold your pleat together till you stitch it to the deck (the part that goes under the mattress) without the risk of a pin falling out.
Step Five: Cutting out the deck.
In a perfect world everything would come out to the exact specification that you intended: the need for a deck piece that is 54" x 28" (remember we are allowing for 1/2" seam allowance). However, the world is far from perfect. And I don't know about you but my sewing isn't always perfect either. So I save this cutting for now.
Lay the fabric from which the deck will be cut out on a large, flat, preferably somewhat hard surface (ie: your living room floor) with the right side facing down. Really any fabric can be used for this. I used some cheap muslin that I had on hand. Then lay out your skirt piece like in the picture below.
Once you are satisfied that your corners are square and the sides are straight, and the inside rectangle is approximately 53" x 27", trace the inside edges with a pen. Remove the skirt pieces and cut the deck fabric 1/2" on the outside of the line you traced (seam allowance remember).
Step Six: Sew(Surge) the skirt to the deck.
With right sides together sew (or surge) each skirt piece to the deck 1/2" from the raw edge.
Step Seven: Put Crib Skirt on crib.
Step Eight: Stand back and admire your beautiful masterpiece!
My first tutorial! Hopefully this makes enough sense...
Step Three: Hem bottom edge.
Your fabric should be folded as in the diagram above only tighter and pinned with raw edges together of course instead of loose. In other words the point at which the two sides should meet is on the line right under the 4. (Does this make sense or is it confusing. I seem to have that effect on people... Confusing them I mean...)
Once you are satisfied that your corners are square and the sides are straight, and the inside rectangle is approximately 53" x 27", trace the inside edges with a pen. Remove the skirt pieces and cut the deck fabric 1/2" on the outside of the line you traced (seam allowance remember). 
