how do i make belt openings that dont look like this

trying to make one of those horizontal slit openings for a wrap skirt belt to thread through. saw them on rtw and thought itd be easy but mine looks... rough i interfaced the area and stitched a rectangle then cut through the middle but the edges are all frayed and messy?? pic attached of my disaster is there a trick to this? do i need to do it before or after sewing the seams? different stitch? help lol