i hardly ever use patterns LOLOL!! and i really hate that all the stupid ready made covers aren't contoured! that just irks me. LOL
so take a look at the short end. you need that curve... so stand the changing pad up on the short end and trace the curve. that will give you the basic shape you need. add 1/4" to the curved side and the short sides for a seam allowance. add at least 3" to the strait long end so it will tuck under the pad. you'll need to cut 2 of these from your fabric.
now you'll need a piece of fabric to cover the top and both long sides. i'd do one piece of fabric. this will be a rectangle shape. lay the pad on the table as if it was on the changing table. measure across the pad from left to right (curved end to curved end) and add 1/2" seam allowance (1/4" for each side). now you're going to take a 3-side measurement: from the back side starting from table, measure up the back side, across the top (follow the curve as closely as possible), and to the table down the front side. add at least 3" to this measurement so it will tuck under the pad. you only need 1 of these from your fabric.
cut your fabric pieces (you should have 3: 2 curved ends and one big rectangle). right sides together, sew the curved ends to the rectangle. then add elastic to the raw edge so it will tuck under the pad.
if you have trouble with the measurements, let me know and when i get home, i'll measure my pad for you. i think they're all standard, aren't they?