Go to the solo tab from the menu, grand prix, choose a category (F1/2/etc) team, driver, track (I chose Monza). Pull down the laps and difficulty if you wish. Then go over to assists and select gearbox to manual.
Starting the race you will need to hold
until the lights go out. For the rest you'll need to use
for going down a gear and
to go up.
Follow all of that and aim for 1st place.