You would think replacing a cracked toilet seat is easy, but nooooo, not for a one piece toilet ! This is because there's no nut on the bottom side, so Kohler & others put anchors instead. Thru the years of water seepage, the two screws will be rusted stuck forever ....

Materials needed:
Penetrating oils like WD40, PB Blaster etc...
Big screw driver.
Vice grip
Small hack saw
#10 ribbed plastic anchor & screw for dry walls
Lots of patients

Step 1: Remove the Toilet Screws

Flip the seat cap cover. If you're lucky enough that the screws turn loose, just remove the screws, put in a new hardware and you're done.

Step 2: Soak the Screws

Now, this is reality talking: most likely yours will be seized like mine .... So spray the area real good with penetrating oils (I used PB B'laster), over night if possible, and keep the exhaust fan on.

Step 3: Brute Force Time

After 2 days of soaking, mine still stuck. So I started cutting the plastic caps until I can pry the seat out.

Step 4: Remove the Screws

Once the seat is gone, you'll have enough room to cut the screws out. In my case, I griped the head and started gently rocking it back/fwd, all the while spraying. You're in good shape if little pieces of anchor metal starts coming apart. With a little luck and lots of patients, it'll pop out in one piece.