I cut my first steek in 2006, but it wasn’t until 2016 that I realized it would be a good idea to knit the button bands on before cutting the sweater open. It prevents pulling on the cut steek stitches and assures you that you’ve got button bands that won’t deform the front edges of the cardigan.

This sweater has special steek stitches that show where to cut, but your sweater doesn’t need to.
A crocheted steek is my favorite. Eunny Jang has great info in steeks. I did a 5 stitch steek on this sweater and am using sticky, woolly fingering weight yarn and a skinny hook to secure it with a crocheted reinforcement.
And on to button bands I go. I reinforce my steek before adding button bands (so much easier without button band flaps flapping around) but I don’t cut it. I don’t know where I got this idea, but it certainly avoids straining the steek while picking up stitches and knitting, and heaven forfend, ripping and reknitting.
In the past, as shown here, I held the yarn inside the sweater and pick it up from the outside. I’d usually start the first few stitches with a crochet hook and transfer them to my needle, then do the rest with the needle. Keep the hook handy, in case you run into a tough spot. If you have a Tunisian hook, you’re living on easy street.

In my most recent cardigan, not shown, I picked up stitches from the front under one stitch and back out to pick up the yarn from the front, much like the crocheted steek reinforcement I did. That worked well and created a nice turned-under steek facing once the steek was cut. Recommended!

After the steek is cut (refer to Eunny’s video linked above for a demonstration), admire how nice your bottom band looks.
You can see how tidy that steek is inside.
You can leave that steek as is, or cover it with ribbon whip-stitched over the cut edge, which I usually do because I love some secret prettiness.