Secure the carpet at the bottom of the riser to the tackless strip. You may need to staple it.
Stretch the carpet by hand to contour the stair to look like the other stairs. If it is french cap where the carpet rolls neatly around the cap you will have to force it into the top of the riser into the tread corner and staple. If it is waterfall just stretch by hand until the carpet is on the tackless on the tread, starting in the middle and working out.
Rent or borrow a carpet kicker and kick it onto the tackless. It is a little awkward, I know. Make sure the teeth of the kicker are set to the height of the carpet and backing, hold the handle down firmly so that the kicker doesn't skid over the carpet ruining the face and injuring your groin.