Measure the length of the top. Measure the length of the pillowcase, then the length of the kid, starting at about the bottom of the clavicle, or where you want the neck of the top to hit. Pillowcase tops are VERY roomy, so going long will give it tons of room to grow with your kid. If you do want to trim the pillowcase’s length, remember that the finished opening of the pillowcase will be the bottom of the top, so trim from the other side. Add 1/2″ to the desired length of the top to account for the top hem before you trim.
If you don’t want to trim the length of the pillowcase, just cut off the seam at the end opposite the finished opening.
Create the armscyes. Take a tank top that fits your child well and use it to trace the curve of the armholes onto both sides of the pillowcase. Remember that the top will hit at about your child’s clavicle (minus that extra 1/2″ that you added), so don’t make the armscyes too roomy, or your kid will suffer a lot of nip-slips.
Double-fold and hem. Fold the top raw edge down twice by 1/4″ each time, then sew down with your stitching near the fold. This hems the top, and leaves a casing for the elastic that you’re about to add.
Elasticize the top. Measure across your child’s chest, then cut two pieces of narrow elastic to that measurement (this is a good use for leftover elastic scraps, since the lengths that you’ll need are so short). Use a safety pin to thread each piece of elastic through the casing that you’ve made for it, hand-stitching it down at either end.
The elastic gives extra room to the neck, so that your kiddo will be able to pull the top on over her head.
Measure the bias tape. Handmade double-fold bias tape is VERY easy, but stash double-fold bias tape or ribbon will also work for this. Take your child’s over-the-shoulder measurement, and add it to the underarm measurement that you can take from the pillowcase top’s armscye. Add 1/2″ to that total, and cut that length from two pieces of bias tape.
Sew the shoulder straps. Align the raw edges of each armscye to the wrong side of each piece of bias tape, so that the raw edge will be encased when the bias tape is sewn together. First sew the raw edge of each armscye to one half of the bias tape, then close the bias tape and sew it shut around the armscye. Overlap the two ends of each bias tape, first folding down the outer end that will overlap the inner end, and sew the bias tape shut all the way around, making a circle to finish the shoulder straps.
You may not be lucky enough to find a hand-embroidered pillowcase, sure, but even a plain one can be embellished, and there’s a good chance that you’ll run into some funky old-school licensed prints. Vintage Scooby-Doo pillowcase top–how cool would THAT be?!?