I want to use this pocket on my apron so there are two ways to do it. Use a seam ripper and removed the threads stitches holding it onto the shirt and save it. The other way is to cut around the pocket leaving 1/2" of selvedge around the pocket. Flip the pocket over and cut out the shirt fabric close to the stitching. This will leave the pocket with the 1/2" selvedge. Press this selvedge under the pocket on all sides.
Save this pocket to sew onto the apron at your desired location. When stitching, you will use the original stitching line as a guide.
NOTE: if you cut the shirt around the pocket, then you will have less fabric to work with later. On this apron, I cut around the pocket to try this and see what would happen. I didn't want to be bothered with seam ripping.
Next, I cut up both side seams to the armpit of the shirt. I did this because I am reserving the front of the shirt for the bib on the apron. If I was making a half apron, I would not cut the side seams. I would use the fronts for the skirt of the apron, leaving the side seams intact.
Then fold your shirt in half down the center back, matching armpits. Locate your armpit and cut straight across the back from armpit to center back fold. This cutting line will be the waistline/top of your apron skirt. The shirt tail is the bottom of the front of your apron.
Cut off both short sleeves along the seam line and reserve those for the ruffle. Go ahead and cut open the underseam on the sleeve and lie it flat for use later.
Use the back yoke of the shirt for the waistband. When you cut the waistband and open up the yoke pieces, you will notice that one of the yoke/waistband pieces usually has nice ironed on interfacing. Use this interfaced piece for your outside waistband. NOTE: I used the waistband pattern piece from Mary Jane's new apron book for illustration purposes. When making an apron for myself, I like my aprons to wrap around my hips to my backside with about a 6 inch gap in back, so I take my waist size and subtract 5" and this leaves me a 6 inch opening after sewing 2--1/2" seams/hems.
Now it is time to work on the bib. NOTE: I again used the bib pattern piece from MJ's new book for illustrative purposes.
Lay your bib pattern piece (or use your own measurements from your waist to just above your bustline...however high you want your bib...plus 1" for seams/hems. In this case, I wanted to make a detachable bib, so I used the front placket for the bottom part of the bib. After you cut around one side of your bib, flip it over and cut the other side. (you can fold your fabric and make a fold line to place your pattern piece on.) I chose to fold the fabric over on itself, using that center button...of three buttons....as my center point, see the photo below too.
Match up the outer two buttons of the three on the placket, then cut your other side of the bib.