I train guards, so my advice may not be the best, but this is what I would do:
Options for training:
1-pos at C:
ID (also trains IS and SB)
IS (also trains ID and JS)
1-pos at PG:
PS (also trains HN and DV)
2-pos at C/PF:
RB (also trains ID and IS)
2-pos at SF/PF:
One-on-One (trains DV, HN, IS, JS)
JS (also trains JR and IS)
You could also 1-pos train SB and HN, but I wouldnt bother, because 1) I dont believe SB has a great impact; the SB pops you get from training ID should be enough. 2) Training PS and 1-1 also trains HN, but training HN doesnt train PS.
I recommend training 1-pos first. Once you get more trainees, you can switch to 2-pos training.
So train 1-pos ID and IS. When you get more trainees, be sure to rotate in RB, along with 1-1 or JS.
PS will be hard to train, because you will need to make a sacrifice. Either you play your trainee in a game with his 3 atrocious guard skills (which will probably result in a loss), or you play him 48 minutes in a scrimmage, giving him less playing time in league games. (If you dont mind sacrificing the complete training of your other trainees, you could also play him at PG for a portion of each game, but this becomes difficult to manage.)
Once your main trainee gets a pop in the skill you are training, switch the training; balanced training is faster.