You need to cut 3 pieces of fabric. The 3 pieces will be the same length and I use somewhere between 40" and 45" for girls 12mos to 6+. Kalla's waist is 21". You want your top tier to be shorter from top to bottom than the second tier. For an idea of measurements I used the following for the girl's:
Kalla size 6~ top tier 7" x 45" cut 1
second tier 13" x 45" cut 2
Maressa size 2T~ top tier 7" x 42"
second tier 7" x 42" cut 2
If the skirt you want to make is somewhere in between, add or subtract inches from the second tier cuts. The top tier includes 2" for the elastic casing. When you've got your strips cut, sew the top tier piece on the sides (you can use french seam here if you don't have a serger for a neat look). Now with right sides facing sew the 2 second tier strips together. You should now have 2 tubes, one about twice as wide as the other.
Now that your side seams are done and finished how you want, you want to take the really long tube (the second tier) and hem the bottom. You can hem it plain, or do a rick rack hem, or use bias tape (like I am above) or contrast fabric hem. The prepackaged bias goes on much like the contrast fabric. The main difference is that it has all these neat little folds already. If you look carefully at your prepackaged bias tape, you'll notice one side is folded wider than the other just very slightly. You'll start with that side on the wrong side of your hem. Open the tape and pin it edge to edge. Sew down in fold. Now fold it over to the front side and topstitch it down.
Now that your hem is done, you'll need to gather the top of the second tier. This is how I prefer to do it (not necessarily the ONLY way). I gather from seam to seam making it easier to get even gathers along the skirt. So you'll start at one seam, backstitch, then turn your stitch dial up to the highest number. Stitch across til you get to the other seam. Now when you pull your skirt away from the machine to cut the threads, cut the top thread short, but pull the bottom/bobbin thread long. This will be your thread you will pull to gather. Now start again at the seam you stopped at and repeat last step to other seam.
Now pull your bobbin threads carefully and gather both sides.
Now with the way I make these skirts, I generally only have one seam for the top tier (you can always cut it in half and sew both sides if you like). So to help me gather each side neatly and evenly to the top tier, I mark the opposite side of the seam to show me where the 'other' seam would have been. I gather and pin right sides facing and match seam to seam and seam to mark.
Now sew carefully across gathers to attach the tiers. Be careful that the fabric on the bottom is feeding flat as it easily catches under and creases.
Here is where I serge that seam. If you don't have a serger, then use a zig zag stitch or over casting stitch on your machine to cut down on fraying.
This part is a totally preference thing. I like my tiers to be topstitched down. So if you like, on the right side of your skirt topstitch the top tier right above the gather.sew a button or bow to the front center, or stitch an initial on the inside center back where a label would go with embroidery floss.
Ta da! Now go applique some cute shirts to go with it! And now that I see this skirt all done, I'm thinking I need to make one momma size!!