Cut a hole on one side big enough for your horse to fit their mouth in. I used a giant tin of milo to trace around. Zig zag or overlock the edges.
Cut out a long rectangular strip of fabric long enough to go around your horsey mouth hole. I used contrasting fabric but you don't have to. Pin right sides together around the hole as shown in the picture and stitch.
Turn the bag inside out and pull the fabric to the inside, pin as shown in the picture and stitch.
Cut out 2 rectangles or trapezoids, fold all the edges down and stitch. Then make 3 evenly spaced button holes or preferable fix 3 large eyelets. I didn't have any on hand so I just made double thickness button holes.
Anyway, after doing all this I had to put it to the test;
I believe it recieved the Charlie stamp of approval! After much thrashing about and holding two biscuits of hay the hay bag is still going strong. Charlie has already learned that it means food time and whinnies at me anytime he sees it.