The simplest pillow covers are ones with envelope-style backs, in my opinion. There's no zippers or buttons to worry about (which take longer to sew), and the envelope style makes it super easy and quick to wash, and to switch out your style or colors with each season. These covers require just 3 cuts of fabric, two hems, and one seam.
The rule of thumb when making a pillow cover is knowing what dimensions to cut from your fabric. My basic rule I like to follow is to cut out an inch bigger than the pillow I'm trying to cover. For example, this pillow insert is 18"x18"; therefore, you need to cut a 19"x19" inch piece of fabric for the front cover piece. I typically use about 1/2" seam/hem when I sew. You can adjust accordingly to the size of seam/hem you prefer to use.
The back pieces are cut so that they overlap each other, like an envelope. This makes it easy to insert the pillow and switch it out to wash or change it up. For this 18"x18" inch pillow insert, I need to cut two pieces that are 19" L x 14" W.