Remove the old high chair pad to use as a template to create a pattern for the new one. Turn one piece of fabric upside down and place it on a table with the wrong or back side facing up. Place the old high chair pad on top of this, near the edge of the fabric, and trace around it with a pen or a pencil.
Cut out the pattern for the high chair pad from the first piece of fabric, then use that piece to trace and cut a second piece from a coordinating fabric. To do this, place the new fabric right side down on the table and put the template on top, wrong side down.
Pin the two pieces of fabric together with the right sides facing one another. Use straight pins positioned around the perimeter of the pattern to secure the fabrics.
Use your sewing machine to sew a straight seam around the edges of your project. Position your seam approximately 1/4 inch away from the raw edges of the fabric. Leave an 8-inch length of one side open, without a seam, so you can turn the piece right side out.
Turn the high chair pad cover right side out through the hole you left. Use your hand to push out the edges and corners of the piece.
Cut a piece of 1/2-inch foam padding to fit inside the cover you have sewn. Squeeze the padding inside the cover through the hole you left for turning.
Use a hand-sewing needle to close the hole in the side of the pad cover.