get a regular sweater
(i used my moms 80's retro print one)

cut out the arms with a tiny indent so your under tshirt shows
( you can use the arms for leg warmers afterwards)

after you cut out the arms measure 1" or 1.5" from the collar and use a pencil or ruler to draw a "v"... the length of the "v" neck depends on how many inches you cut each side

make sure you lift up the neck part and cut only the front not the back

then just accessorize!! i used buttons for mine!