Step 1: Start a career with Oguri Cap or Mejiro Mcqueen.
If you dont have any of them then you can do it with Mayamo Top Gun or Haru Urara, but it will take a bit longer.

Step 2: Train speed with the appropiate amount of rest until you reach the debut race.
You can train something else, but usually the only important stat on the first race is speed.

Step 3: Win the debut race.
If you dont win, then try again the next day. You can skip this if you already did the win a race daily mission.

Step 4: Spam rest until you fail the career mode.
During the career mode of these umas, the next goal after the debut race is to earn a certain number of fans, so you will fail if you dont run any race after the debut race.

Step 5: Finish the career and earn your daily carats.
You can start a new career for tomorrow so you can start repleneshing your energy.