Mattress stitch – step by step :)
My needle is quite long, thin and a bit bent (that comes from the many stitches and is in this case an advantage; o)), I take the thread twice …
 … here I want to close the opening at the top of the doll’s head …
 … I fold the tricot fabric inside …
… from the middle I start to sew …
… for me it is important that the mattress stitch of the size of about the stitch of my sewing machine corresponds, so that the doll is sewn evenly at all points, that is about 2.5-3mm …
always use the “ribs” or “rows” of the knit pattern in the tricot fabric …
Stab with the needle every two rows …
… then I draw the thread about every 2cm and make sure that the tricot fabric stays nicely inside …
and that’s how my mattress stitch looks when he’s done: o)
 here are some pictures that show how I sewed the head with the neck in a hand puppet and thereby “disappear”
 and that’s how it looks …
Source: HERE