You’re just going to knit it back and forth without doing any increases or decreases until it measures a certain length. Follow your pattern. If it’s in stockinette stitch, knit the knit rows and purl the purl rows. Sometimes you’ll see this written “knit evenly” or “knit straight”.
One thing I would suggest is that you mark the place you begin to knit straight on the edge of the piece with a marker, bit of scrap yarn, or safety pin. Mark it also on the other side. You’ll need to make a corresponding edge on the other side only in reverse. This will help you to easily see where you need to do this. Sometimes I’m sure I’ve measured correctly, but I end up with one shoulder higher than the other. This is why I say mark the spot.