On upholstery, it's customary to cut with the grain of the fabric on cushions running front to back. I'm assuming that by "long", you mean front to back as it sits on the chair, and "wide" means side-to-side?
So, if I've read that correctly, with a fabric width of 54", you are going to easily be able to cut the cushion covers parallel on the lengthwise grain. Cut right down the foldline to simplify things; that will give you side seam allowances of 1.75" on each side.. If you are planning to only have one seam at the rear of the cushion, each piece to be used for a cover is going to be 64" long.
You will also need to cut 6" by 34" strips in the same grain direction, 2 for each cushion, to fill in the sides. (These are fairly generous seam allowances, which is good if you are not really skilled, so that if you end up a bit crooked you'll have wiggle room to fix it.)
Get some chalk and mark the fabric with the actual size of the cushions measured out in the center of each piece of fabric, those will be your stitching lines. (You do this by folding the fabric around the cushion, and then running the chalk along next to the foam on the wrong side.) The easiest closure method for an amateur is probably going to be velcro attached with fabric glue on the back seam. Foam doesn't squish all that well, so you are going to have to extend the opening a bit onto the sides of the cushion so that you can get it in and out -- well, unless you are going to hand-stitch the final seam shut around the foam. (If that is your plan, be sure to buy some curved upholstery needles to do that part; a straight needle will be a major PITA.) Simplest way is to use a bit of extra velcro and close the opening like a box, folding in the side wings and then velcro-ing together the finished long edges over them.
BTW, if the fabric isn't finished with vinyl on the back surface, invest in a pinking shears to cut it; otherwise you may end up with some nasty raveling on the edges.
