So, a quick how-to for you! This tutorial will explain how to convert the gathered waistband of a regular skirt pattern (or pants for that matter, now that I think about it), into one using "buttonhole" elastic, rather than regular elastic. It's really very easy.
Any pattern with a waistband will do, and you should be able to follow the instructions along. This one is headed for my niece Lillie (hence - the Lillie-Billie on my fabric in case you were wondering!)
Here we go...
Follow your pattern instructions until it comes time to sew your cut waistband piece sides together.
Take each piece - fold it in half lengthways with wrong sides together - and give it good press to create a nice crease.
The crease is going to be the top of your finished waistband. Place the two pieces right sides together, and you are going to sew as per your original instructions - EXCEPT - we will leave a gap for the elastic to thread through, near the top of the inside of the waistband.
Start at the front of the waistband - sew down to the crease, keep sewing for 2-3 extra stitches past the crease. Now backstitch to reinforce, and cut your threads. Skip down to leave a space the width of your elastic (mine was 3/4 inches wide). Start sewing again - all the way to the end.
Closer up - it should look like this (except I've flipped it over to the other side in the photo - sorry!)
Repeat for the other end of the waistband. Both elastic holes should line up on the same side of the waistband. If they don't - then you've sewn one on the wrong side! Don't worry - just unpick and do it again. :)
Turn your waistband the right way through, and refold at the crease.
Take your elastic - and cut it about 2 inches longer than your waistband piece. It just needs to be long enough to sit nicely when unstretched.
Next, take your elastic ends and turn under just a little on each end. Sew over them just to neaten and so they don't unravel (as in the photo below).
Take each end, and push them through the holes you made before. I use a safety pin to secure them on each end. You can thread the elastic into the casing later if you prefer - but it is so much easier to do it now while the waistband is not attached.
Now - attach your waistband as per your pattern instructions. Then sew a casing, just along the waistband back where the elastic is. You want to push the elastic up to the top of the waistband (I usually topstitch first as in the photo), then sew your casing just in from the side of the elastic. This keeps it nice in place, and stops twisting, etc. Be careful not to sew into the elastic! Otherwise you won't be able to tighten it.
You will end up with two ends that look like this:
The Left: 
and Right:
With the casing in between.
Lastly - you want to sew a little button near each elastic end (pretty much where my safety pins were)...
Then give the elastic a bit of a tug...push the button through once..
...and then back again to give a nice finish:
Once you have done this on both ends - you are finished! Carry on sewing the rest of your pattern to get an adjustable waist skirt like this:
With a smooth front
..and an elasticated back:
Any questions - feel free to ask!