I don't have a digital camera and my polaroid won't take clear shots of close-ups for me to scan. So, here's a drawing of a machine head and where the tack would be in relation to the needle. I put tape over the point of the tack to hold it down to the machine bed.
Practice with a few scraps of fabric. Make the fabric about 6" square and put the center over the tack (if you have the tack 2.5 inches to the left of the needle--use a ruler.) Start stiching with a straight stitch and your feed dogs will pull the fabric and stitch a circle.
You can also just draw around a round shape (like a small plate or lid of something) and then stitch along the lines. That works just fine and is how I did my first few.
You know, they really don't have to be round. You could make squares and just round off the corners a bit. That would work fine. There are all kinds of knits that wouldn't even need sewing if they were a bit thick. Sweatshirt fleece or evey poly fleece can just be cut and used without stitching the edges.