Open the stitches where the collar band meets the placket, on both sides. This gives you access to the inside of the placket. Try to keep the end stitches intact, they'll help when you sew the shirt back together.

Slide the stay down into the placket, between the button/buttonhole and the outside edge of the shirt. Leave the small hook exposed at the top so it sits inside the collar band. You'll sew right through that hook in the next step.

Stitch through the stay below the hook with two to three stitches maximum. Any more and you risk "cutting" the material. Repeat on the other side.

Look closely at the inside of the placket. You'll see a row of stitches running along the edge, often in a slightly different color than the shirt itself.

Use your provided seam ripper to gently open a few stitches, just enough to slide the stay through. Don't open the whole seam.

Lay the stay next to the placket. Mark it with your finger or a pen, then cut. Leave it slightly shorter rather than longer. You don't want tension once it's inside.

This is the detail that matters most. A sharp edge can rip the fabric over time. A rounded corner prevents it completely.

Slide the stay through the opening, keeping it along the outside edge of the placket, between the buttonholes. Push it all the way up.

Tuck the top of the stay so it hooks around the top buttonhole. That's the anchor. Once it's seated, it won't fall out.

Same process on the opposite side of the placket. Open, size, round, insert, hook. You're done.