Mark the sticks where you need to trim them, I made mine just longer than half. Glue the chair together by fixing one half a popsicle stick across the bottom of all the sticks at the back.
This will support your chair arms so you want it to stick past the edges just a finger’s width or so! Glue it in place and trim on an angle.
You want your seat to be as wide as the bottom of the chair back, so I used the chair back as a guide to mark my seat slats where they needed to be trimmed.