You need to buy 1/4" thick solid trim stock. It can be ripped to widths needed for your doors or windows, or purchased as 1" stop material strips. Available in 3" widths in hardwood or softwood, the material should be glued and finish nailed on existing jambs to bring trim flush to walls before applying casings. Paint or stain to match. You can find this material in any home improvement wood trim dept, or other locations that sell trim.