I'm sure you all could figure this out, but I thought I'd make it easy on you by giving you all the measurements that you need!
I used my daughter's old crib skirt as a sort of template, but this one is for a boy- less girly! Or if you don't have a crib skirt to look at, here you go!
A few things first:
1. I wanted the hanging panels of the skirt to hang straight. I've included measurements if you decide you want it to be a little ruffly.
2. Also, on the long sides (not the short sides) I added a pleat. I'll show you how I did this, and you can also do it on the short sides if you want. Or you can just have all sides hang straight! I've got measurements for that too!
3. My daughter's crib skirt was WAY too long and didn't look so nice once we lowered the crib mattress. I made this skirt so that it will hang ever so nicely once the mattress is on it's lowest setting. So, length is up to you.
Let's get started!1. Wash and press your fabric. I like to use
lots of starch! It keeps things nice and crisp.
2. Cut the deck of the bed skirt (the part that the mattress will sit on) 28"x51" just like the picture below!

2. Here's where the options come in:
option a: ALL hanging panels to lay
flat1a. Cut 2 pieces for the long sides 51"x13" (On the crib, I measured from the
bottom of the mattress to the floor and added an inch (12+1=13") - you can
customize to your own crib by doing the same (if you do, substitute that number
in for the 13").
2a. Cut 2 pieces for the short sides 28"x13".
option b: ALL sides to have a
pleat in the center1b. Cut 2 pieces for the long sides 55"x13".
2b. Cut 2 pieces for the short sides 33"X13".
option c: ALL hanging panels to be
ruffly:
1c. Cut 2 pieces for the long sides 77"x13".
2c. Cut 2 pieces for the short sides 42"x13".
You now have 5 pieces ready to go!

3. On
each hanging panel: serge (don't trim any fabric off) or zig zag stitch
both short sides and
ONE long side. You should have only
ONE raw edge on the long side of each panel when you're done.

4. On the
long, serged side of each hanging panel, fold over 1/2" towards the wrong side of the fabric. Press. Pin in place and sew that baby down! Then do the same on the
short, serged sides.

Three sides of each hanging panel should be beautifully hemmed, with still that one long side on each raw and untouched!

5. Sewing the panels to the deck! Here's where all our options come in again.
option a: All flat hanging panels
1a. On your deck, find the center of each side and place a pin there.
2a. On the raw edge of each panel, find the center and place a pin there.
3a. With right sides facing, line up the center pins for each side and pin together.
option b: Pleats (we'll do that below)
option c: All ruffly hanging panels
1c. On your deck, find the center of each side and place a pin there.
2c. On the raw edge of each panel, sew a basting stitch (be sure thread tension is set
low) about 1/4" away from edge.
3c. On that same edge of the panels, find the center and place a pin there.
4c. With right sides facing, line up the center pins for each panel and pin together.
Pull on the bobbin thread of your basting stitches to gather the panel into a
ruffle that is a 1/2" shorter on each side than the length of your deck. Pin in
place.

There will be an extra 1/2" of deck fabric on each side of hanging panels which is why it is VERY important to line up those centers! Sew each panel to the deck using a 1/2" seam.
Option b: Pleats!
1b. On your deck, find the center of each side and place a pin there.
2b. On the raw edge of each panel, find the center and place a pin there. On the RIGHT SIDE
of your fabric, measure 2" to the left and right of your center pin, and mark with a pin.

3b. Bring the left side pin towards the center pin so that there is now only 1" between pins,
folding the fabric over. Pin in place. See picture below.

4b. Do the same with the right side and pin. Baste the pleat in place.

This is what your pleat should look like:

5b. With right sides facing, line up center of pleat with center pin on deck. Pin along the raw
edges and sew with a 1/2" seam.
6. Serge all four raw edges together.
7. Press the seam towards the deck and pin in place.
8. Be sure to fold the corners in nicely!
9. Top stitch that seam and there you go! All done!
You can see, I did a pleat just on the long sides.
I'm no expert! Let me know if anything is confusing and I'll try to explain it better. Also, let me know if you make a crib skirt of your own, I'd love to see it.