as the title says

Like all other riders, too. Give him a trainer and train him in stage race, sprinter, puncheur, time trialist, where he is the strongest. Depending on the speed you picked at the career start it takes 1-4 seasons until he is a solid rider.

what type of training do you do when you want them to improve their downhill stat?

You can´t train that, it gets automatically trained.

and what about flat, is that the same too?

yep, only TT, mountain, hill, tours, sprinting, northern classics, puncheur. The rest goes automatically.

Flat can be trained faster, e.g., in Puncheur or Sprinter versus Tour or Mountain training. The only skill that cannot be trained is downhill (and fighting)

You can also look up the training types in STA_training_excersise. Things with a number get extra training, eg sprinter training doesn't train climbing extra, but climber training does.

Flat is best trained by training Fighter. Normally used for riders that you want to be lead outs for sprinting.