I don't know of any directions to do such a thing. If I understand correctly, you want to make a pullover sweater into one that is open at the front (maybe with buttons, maybe not). I believe it can be done, but I'm not sure you would be satisfied with the results. If it is a sweater you made and still have leftover yarn, the results might be passable. But if it is a purchased sweater, chances are that you will never be able to match the yarn and would end up coordinating with some other yarn instead.

I think that you would first have to make a line of stitching with a sewing machine just to either side of the center front (I would suggest placing tissue paper at least underneath the sweater and possibly over the top as well). After stitching, cut right down the center of the sweater, then pull off the tissue paper. You could then pick up stitches along one of the front sections you just made and knit an edging. I would think you should make it twice the width you want, because in order to achieve a finished look, you will need to fold it to the back and hand stitch it down over the cut edge. Do the same for the other front side. Of course, if you want buttons, you will need to knit buttonholes in the appropriate places.

Unless your sweater is made with one of the thinner yarns, I don't think you'll be pleased with the results.

There may be other ways to do this, but I don't know of any. I hope someone smarter than I will answer this question for you. Good luck.