Wing it... I did a supper simple doll poncho with out any patter at all. but if you really need one here gose.
kfb first and last stitch of every row.
When you have it as long as you want it when folded in half. Cast off 40-45 stitches in the midle and finsh knitting the row like normal.
Okay k2tgh at the start of the next row knit tell you get to where you cast off... Cast on the number of stitches out cast of and finsh the row k2tgh at the end of the row.
K2tgh at the begining and end of every row tell you have 2 stitches left on you needle cast off weave in ends..
Fold it so the slit is verticl it makes a nice V neck.
That is a picture of the toy sized one did. In my avitar.