I've used this trick about a million times. With a pashminas or other fabric scarves, making an infinity scarf is as easy as one line of straight stitching. However, with chunkier knit fabrics, the process of becoming an infinity scarf is a little bit different. This scarf has probably been at my parent's house since the dawn of time (well, maybe more like 10 years). I love the fact that it's handmade, but it was a little too short to be worn as a traditional scarf, so I never wore it. But in less than 5 minutes, I turned this scarf into a piece I'm going to wear again and again this winter! Here's how---
Knit Scarf of your choice
Any size needle
To start, fold your scarf in half lengthwise and pin the two finished edges together. Thread your needle using a length of thread about the same length as one of your arms. Once you have the thread through the eye of the needle, match the two ends of the thread and tie a knot. Next you'll need to start your stitch. The knot in the bottom of the thread will just pull through a chunkier knit, so this requires getting a little creative.
Start your stitch by inserting the needle through one edge of the pinned scarf ends, but do not pull the thread the whole way through. While holding the loose end, start another stitch with your needle. Once you've create the loop for the second stitch, pull the loose end through and tighten, forming a new knot. Depending on the size of the knit, you make have to do with twice to secure your thread. If you are using a matching thread color, you don't have to worry about hiding your knots-- my scarf was a bit of trial and error but all of the stitching is basically invisible since I stitched black on black. Continue stitching the two scarf ends together using a whipstitch. Once you get to the other end of the pinned scarf ends, recreate the knotting technique used in the beginning to finish your stitch.
Easy as pie!