What must you do in order to acquire a S+

A lot. And on a champ like Swain that can dominate, even more. You probably want to pay special attention to:

1. CS. Swain can easily get a lot. So to beat everyone else, you may want to cheat and get a zz'rat portal (note: this isn't a good way to play Swain, but it is a good way to try for an S)

2. You need credit for almost every objective taken. I'd set goals. 3 Dragons (that you're involved in taking). 3 to 5 structures. and at least a Baron.

3. Again, rig the system and get one of the +2gp10 items. Not because it works for swain but because gold matters. Cheat to win.

If you're asking in general, and not just on swain, play less popular champs.