When I start mattress st with the tail of yarn at say the side seam of a sweater or of a sleeve, I go throught the opposite side and back to the original side and then put the yarn through the loop that is formed. That does effectively make a knot which secures the beginning of the mattress st. For the end, I just weave in the yarn into the mattress seam but I suppose you could also make a single knot here too before weaving in the end.
I am wondering where I might view of video of how to do mattress stitch? I've always used back stitch up to now. My daughter-in-law said that she uses mattress stitch in sewing up her sweaters so I want to try it. thanks for any help.