Okay, taking a closer look at this pattern. This slipper pattern has been around for a long time, at least the 1940’s from what I’m reading. I remember doing these for the 4H Club project in the early 1960’s. Here’s a website with the exact same pattern with pictures that will better explain how to do it. This is an oldie but a goodie pattern. It’s great for quick Christmas gifts.
Edit: Looking at this pattern, though, they adapted it by adding the toe decreases. I wouldn’t decrease, just keep doing the ribbing straight, run the stitches through with a yarn and tapestry needle, draw it up, and sew up the toe seam. They’re putting more work into this classic than needs be done for a beginner.