You will need a pair of old jeans, a sharp pair of scissors, pins, some lacy or frilly trim (maybe cut off a duvet cover or a curtain?) and a sewing machine. I have used a zigzag machine but straight-stitch will do fine.
Hold the jeans with the zip to the front. Cut just below the waistband, through the zip (avoiding teeth) round to the side seam, cutting straight through the front belt carriers if necessary; they’ll fray nicely.
Then cut down the side seam to just below the bottom of the front pocket; repeat on the other side. Then cut across the side seam and in a curve down towards the crotch, staying well clear of the pockets, and up again the other side to the point just below the front pocket join.
Fold the result down the seam and neaten the two sides so that they are roughly equal. Trim off the side seam up to where the waistband joins onto it, to reduce the thickness.
Then cut up the middle seam on the lower side (you’ll see what I mean) some way & slide that side under the other; keep cutting up the seam until the two sides will lie flat. Stitch down; this is where zigzag is most useful! But you can stitch over the top end of the cut by hand, if necessary, or stitch something else over it. Then trim the excess fabric off underneath.
Take your trim and attach with pins to the lower edge of the apron at the back, with the upper edge about a centimetre up from of the lower edge of the denim. Then stitch it down, starting with one end at the waistband, all along the length over the upper edge, round to the other end, then go back over the line of stitching again. Go carefully over thick bits like seams. Check that the trim is firmly attached all the way round, and that you still have that lower centimetre of denim clear at the front, then tie your thread off or stitch the ends down.
Then, using the tips of the scissors, snip into the denim edge at roughly ½cm intervals all the way round, up to but not into the stitch line. (If by any awful chance you do snip into it, just run a few more stitches over it to hold it) This will cause “chenilling” when you wash the apron, i.e. deliberate & attractive fraying.
You can add a bib using one of the legs; use the seam with decorative stitching (usually the inside leg seam) as the centre, and the hem as the top. Stay-stitch down the sides, then chenille, and stitch to the wasitband of the apron. Stitch the inner front pocket onto one side this, and wind any extra trim into a rosette for the other side. Or use pockets cut from other pairs of jeans, if you need more pocket space! Use the overlocked leg seams as ties; if you have others spare, you can plait or braid them first for a stronger tie. Or use other bits of sturdy fabric as a trim & for pockets – you are limited only by what “waste” fabric you can lay your hands on!
There are lots of other useful things you can make with old jeans – it’s a wonder anyone throws them out, really…