I will assume you know how hard is to train a player!
If you know that, you can pretty much imagine all the different kinds of train. The question will always be how good you want your players to be and how good they have to be! The best way to address these two questions is to look into your objectives and your league! Plus you need to realize how close or far are you from reaching the maximum player salaries without loosing the weekly profit!
So, if you have all this stuff in your mind already, let's look into your example a bit more in detail. You want to train SF and you're telling me that you want one with great potential. There are two points already here to be considered. The first one is, will I be able to buy a player, suitable for SF, with great potential. The second one is, will I be able to train him well and keep my economy healthy!
The first one is easy to answer and you just need to look into your budget, the second, and if your plan is to train him only for two seasons, is also pretty easy as he, most probably, will not be over 30k (depending on the train of course). Of course, you should also keep in mind that you will train other players at the same time and, that the main goal of all the training programme should be: Improve your TEAM!
And I say this because I know a lot of cases of people that want to train a star for the U21 or whatever and they end up selling them because they can't afford the salaries. And the best they can come with is "but at least I made a lot of money". Well.. I say money is useless if you're just waisting your time. You get a lot of money and you have three main options: "should I buy two/three good guys (porbably less good than the one I sold) and keep them?"; "should I buy really nice and young trainees and make them superstars?"; "or should I do a mixture of both?". And sincerly, you are just in a loop because with the first option you don't have proper trainees for your team, in the second one you're going to make exactly the same mistake (i.e. sacrifice the team to train players); and in the last one, although is the best option, you still have to loose time again to train players! And meanwhile, the objective of making a good team and go up in the division is just disapearing!
I don't want to be show off, but when I started playing I was like most of the people here. I was addicted to the transfer list and I would buy a bunch of players that I thought would be better than the previous ones and just continue with that. I was promoted in my first season and I start realizing how bad my players were because I was just taking them out of the train if another trainee seemed better to me. Luckly I bought some young players, I was relegated that season back to the IV division but I kept on training them. I was immediatly promoted, without any defeat but in the finals, and this season, back in the III, I am still training them and going full speed to the II division. There's always a chance that I will not make it but hey, that was not even my main goal to start with!
So to finish all this theoretical talk, I am not going to give you a plan on how to train your players! But you should think first how do you want them to look like in the end and if that suits your objectives!
Cheers