Sash Window Stop

The stops have flexible clips to hold them in place. The arrow indicates where the edges are narrower and will fit in easiest - start insertion at that end (up).

As these are meant to go into the outside slide on windows, you'll need to print in ASA (which is UV stabilised so won't degrade outside).  As ASA can warp during printing, you may find the ASA printing skirt will help with bed adhesion issues.