So far what I can gather is,

You train players daily for bonus.
4- defenders for defence boost of 2%, 4%, 6%, 8%, 10%
Same for midfielders and so on. If you click on bonuses and click on one of them it explains the required training.
8 players for conditioning bonus.

2% will be deducted daily so it's best to train daily in the bonus you wish to use.

It can be a lot of effort but if you plan on using just one or two just train the required types.

These bonuses help you in the matches if you select them live in the match.
If you don't have many green packs to heal before match then train reserves.
Don't just train one thing. Use different training schedules.

I hope this helps.