My SIL is big into the holidays - she always goes all-out with the decorating, creating themes and generally making a big fuss. Christmas, Halloween, New Year's Eve, Easter, Fourth of July, birthdays - all are cause to get the house dressed to the rafters, and the people, too.
I've noticed that aprons seem to be coming back as a trend lately, and she has a few themed aprons, so I thought she might like a nice picnic apron to wear for their many backyard cookouts. Add to my inspiration a neat black and white checked shirt in the donation bag, and this apron was born.

Modeled by my nifty dress form (with my favorite Tshirt preserving her modesty), this apron looks longer than what it actually is, although it is longer than a standard apron, falling to below the knee. My SIL is pretty tall (taller than me anyway) so I made this a bit longer than I would need it, using the whole front of the shirt from just below the collar, all the way to the hem. For a shorter person or just a shorter apron, you could very easily adapt this to be a bib apron, with a tie around the neck and ties added at the lower edge where the sleeves are cut off.
Anyway, to make this, all you will need to do is find a suitable shirt. I chose this button down with a collar and a pocket (because aprons need pockets), but most any shirt should do.
First, iron the shirt. I hate ironing, and avoid it whenever possible, but this will make your job much easier.
Now, cut the shirt straight across as close to the neckline as possible, without involving the collar. Then cut off the sleeves along the seams. You can pick out the side seams, but this shirt was very ruggedly constructed, with jeans-style side seams, so I just cut them out, as well.

Hem your raw edges. I used a plain old ruler to make where the shoulder seams were cut out more angled rather than curved, by measuring an equal distance from the buttons to each shoulder seam and marking it, then cutting off the excess and folding it over twice and hemming.

I made the ruffle by cutting two 3" wide strips from the back of the shirt and sewing them together to make one long strip. I then hemmed the bottom edge, ran a basting stitch through the top, and ruffled it as I pinned it to the bottom hem of the apron. (Here is a great tutorial for making ruffles. It's sooo much easier than it looks!)
All that was left to do after that was to fold over about an inch of what used to be the neck of the shirt, and sew all the way across to make a casing for the waist tie, thread the tie through the casing (I used a piece of salvaged woven cotton tape, hemming the ends so they wouldn't ravel) and voila! new apron!
If you want to go the extra mile, you can add an appliqué, like I did. The fabric reminded me of a picnic blanket, so I decided a watermelon would be perfect. I just cut a triangle out of red craft felt, a semicircle out of green felt, and sewed them to one corner of the apron where it looked right. Of course, it wasn't until after I had folded it all neatly, tied it with its own strings, and taken this picture
Hope this is inspiring, and if you make a shirt apron using this idea, I'd love to see a picture of it!