A training plan is simply a predetermined project based on the training of players . There will be :
At the beginning you will make simple : you'll use only one (primary) training in your first plan.
Success depends on :
Choose a type of training, recruit according to the chosen one and don't move off it for at least 2 seasons... unless you know what you are doing
You need to train effectively ! So don't switch 2 weeks this then 1 weeks that etc.
Ban "general (form)" training or any diluted trainings like "shooting" or "defensive positions"
If you don't really know what to do then choose "Goalkeeping" that will be the easiest to implement
Train young players : not over 20 years old at beginning of their training, and if possible less than that. 17 and 18 years are preferables, 19 years old maximum if possible.
If you run out of money train "old ones" temporarily, but your only thought will be replacing them by young ones soon.
Train players with a correct starting level (passable minimum, solid if possible and if you are wealthy enough) in the primary trained skill (ex : playmaking for inner midfields, scoring for forwards, etc.)
Train the biggest possible number of players, or not far from it, using appropriate tactics/line up.
Think to use all your training slots while playing friendly matches. Double up your training capacity organizing friendlies.
That's up to you.
In general avoid selling before reaching outstanding (10/20) level. Sell before reaching supernatural (14) to not "kill" yourself with too high wages that would reduce your benefits.
You'll prefer selling your secondary players : the oldest ones with lowest secondary skills. You'll keep the best ones for your team.
With generated money buy prioritary new young trainies of higher quality, and good players to strengthen your other squad lines (these players can be older and experienced).