How to attach thread to a crocheted surface

When you are crocheting an amigurumi toy you almost always need to attach a decorative or strong thread to embroider some face features, tighten fingers or a mouth and so on. Many patterns instruct, "attach thread to the middle of the face and backstitch the mouth", or something like that. But it is not always obvious what "attach" means and how you are supposed to do in neatly. 

Download the photo tutorial for 3 simple ways to attach thread to a crocheted piece. Use and share.