Always keep your scissors perpendicular to the table surface. If you angle the scissors, then your cutting will not be as accurate as humanly possible. That may not create problems with this project but when you are making tailored garments, a slight measurement change will magnify itself so it is best to start off being in the habit of making totally accurate cuts. You can do this by making a small nip in the seam allowance or using a fabric marking pencil. First step is to thread your sewing machine and have a new, sharp needle in place.
Needles do get dull and are the most common cause of machine malfunction and poorly formed stitches.
If you do not have the manual to your sewing machine please refer to Locating A Sewing Machine Manual or ask for help on the forum . There are many teachers, with all kinds of machines, out there waiting to give you a hand. Most sewing machines have many different sewing machine feet. If you have not used a foot before it is always a good idea to practice on scrapes of fabric before starting on your actual project. To sew the hems of the pillow cover you will want to make your stitches and even distance from the end edge and as close to the unattached fold as possible.