There are many good instructions online for producing buttonholes in doubled over knit bands. This version is from the Brother Knitting Techniques Book, now downloadable for free online
Many hand knitting patterns are published, often in garter stitch, using slits that one may think of as larger “buttonholes” to create a range of interesting fabrics. Trying to produce such slits single bed, without the use of additional strands of yarn and in turn, having yarn ends to weave in, leaves few options.
This method may be used when creating multiple slits across a row as well. Holding is used to break the knitting into segments. The drawn illustrations show steps are taken, not needle positions.
COR, for the bottom of the slit: 1. transfer the first stitch in the buttonhole group onto the adjacent needle to its right
5. the forward stitch (red) is now knit through the one behind the latch
This essentially binds off a stitch. Repeat steps 1-5 until for the number of bound off stitches required; the last stitch in the group is then transferred onto the adjacent needle to its left. The bottom edge of the “buttonhole” is now complete. To make the top edge of the buttonhole/ slit bring its corresponding needles out to hold, cast on the desired number of stitches with latch tool from right to left. In order to best accomplish this with COR set KM for holding, push empty needles back to A position, knit up to now empty “buttonhole” needles bring empty needles out to hold insert latch hook from back to front through below the last stitch now knit on the right twist tool clockwise bring empty needles out to hold come up between the first 2 needles on the group’s right continue with latch tool bind off, the last loop in the chain is hung on needle already holding 2 stitches from the last bind off transfer tighten up loop
return “buttonhole” stitches into needle hooks, back to B position
COL: set KM to slip in both directions and move to right COR: cancel holding, adjust tension, knit across remaining stitches to LCOL: cancel slip <–>, continue knitting