Hold your shirt up to your body and mark where the neckline will be.You can skip to step 3 if you're happy with the shirt's neckline.
Then lay it on a table(or bed/flat surface) and finish marking the neckline.
Now mark your sides.It's okay if it's too high,you can always cut it lower.
Now flip your shirt over to the back side and mark where you'll cut the back.
Find where you'll tie it (laying on it can help) and draw the strings.
Now cut the apron out! On the back,you'll be cutting off everything EXCEPT what you marked.
Pull the strings and edges so they curl
If the hole for the head came out too low,cut the back of it in half so you can tie it.
tada! an apron!