The simplest is to do a YO in your garter border. Nothing more. Simple!! On the next row, just knit that YO, and you will have a hole… enough for a button. It all depends on the size of your yarn, and size of buttons.
If your knitting is fairly open, you may not need to make a buttonhole. Just poke the button through the stitches and it’s done! Even simpler!
And finally, you can crochet a chain along the edge, but leave a “loop” of chain stitches where you will be placing the buttons. Ready-made button loops!