My favorite knitted buttonhole is the one-row horizontal method because it produces a much neater finish and it is one of the sturdiest of all buttonholes because it does not require additional reinforcement.
Just follow these simple steps and you’ll never make a messy buttonhole again!
Step 1: Work to buttonhole location.
Step 2: Bring yarn forward and slip the first stitch on the left needle purlwise.
Step 3: Bring the yarn to the back.
Step 4: Slip the next stitch on the left needle purlwise to the right needle.
Step 5: Pass the second slipped stitch on the right needle over the first stitch (as if to bind off).
Repeat Steps 4 and 5 until the buttonhole is the right size.
Step 6: Slip the last bound-off stitch to the left needle.
Step 7: Turn the work and use the Cable Cast-On method to cast on one more stitch than you bound off. (When you work the Cable Cast-On, if you insert the needle purlwise instead of knitwise between the stitches, the top edge will better match the bottom edge of the buttonhole.)
Step 8: Turn the work.
Step 9: Slip the first stitch on the left needle to the right needle and pass the extra cast-on stitch over the slipped stitch. Work the remaining stitches on the needle. The photo below shows the horizontal buttonhole.