What you need...
Needle and thread
Buttons, ribbons, embroidery thread/wool (as desired to create the face and collar)
Create a ball of stuffing about the length of the foot of your sock and twice the width, stuff. Roll the sock to create a 'body' shape. Fluff and add more stuffing if needed to create desired shape.
Take a smaller ball of stuffing about 50% bigger than the heel of the sock.
Stuff the head and fluff and roll until desired shape is achieved.
Stretch the back of the sock hole over the front and make a stitch. Make sure you have an overlap of about 1cm.
Stitch with this overlap about a third of the way towards the 'ear tip'. Then slowly reduce the over lap so that by the time you get to the ear there is none left.
Decorate the face as you like this one is done with embroidery thread but you could use buttons, felt, squiqly eyes, whatever you can think of really. I did this freehand but you could draw the features on and stitch over or just draw them on with fabric paint.
Give it a collar, ribbon or elastic is good for this, if you want.
Your cat is now complete. Time to take it on adventures or to let it run free!!
If you make your own sock cat send me a picture at firstname.lastname@example.org. I'd love to make a gallery.