I made a pillowcase dress for my girlie using this pattern from Little Big Girl Studio. There are tons of tutorials though, using an actual pillowcase, or just cutting a rectangle shape from your fabric.
|Ignore the crazy hair... she wasn't into being the model and I only got 1 shot! and if you don't look too close, you won't notice that I had the fabric upside down when i cut it out... Doh!|
There is a printable pattern to make things really easy (though you can just cut a rectangle as well). It is actually a little big for my L, and if I were to make it again, i'd take a few inches of the width probably, but she wears it and it doesn't look too oversize! I'd guess the pattern would be a better fit for a 5t plus.
I also made my own pattern off an old outfit of L's; the goal is for it to fit 12-18 months, but I didn't have anything but a romper to test length off of (I didn't feel like digging forever in those bins in the attic!) I'm not sure on how the length will work...
Got to wait for new baby neice to grow up a bit to see how it fits! These are sooooo simple to do, you can make yourself a dress for so little $$! L's was made out of a half yard, and the little one was a remnant I found at walmart. I don't even know how much fabric it was but I know it wasn't much! So if you sew even a little, and want an easy dress to start on, this is the one!