From UGC HL S13 to S15 my practice for sniper looked mostly like this.
5 to 10 minutes of mouse control practice using either of these sites.
http://www.aimbooster.com/
http://aim400kg.com/ea/
it helps in getting you warmed up and help to efficently hold/move your mouse
I would then play on tr_aim_training (only a couple times a week)
I would go to this room http://gamebanana.com/maps/104304
And stand on the blue dot furthest from the wall. I would put my crosshair on the red square and flick to each blue, green, white and then red square from that center red square (going much further than this is kinda pointless cause your crosshair should be a lot closer to your target than that otherwise you're doing something really wrong.) I would do each square until I hit them 5x in a row.
Ex. Flicking from the red center square to the white square immediately to the right. hit, miss, hit, hit, miss, hit, hit, hit, hit, hit move on to the next square (I would work counter clockwise but that doesn't matter much)
In s15 this usually took me about 20minutes to do (I only did this practice like twice a week cause it's boring as fuck, highly recommend a good music playlist and turning in game sound off)
I would then pub on my favorite servers Skial Badwater US or Skial Badwater LA if those were full I would play Skial Upward TX. For however long I wanted to. (Usually about an hour or 2)
Occasionally I would play SvS servers they help a little bit in dealing with other snipers, and in warmup as well but it's probably the least efficient way of getting ready for 6s.
tr_walkway was alright if you wanted to practice tracking or aiming without scoping in.
And then of course scrims/matches. Usually I played in 4 to 6 scrims a week.
As for the "fastest" probably depends on your skill level but just do whatever you enjoy and have fun doing at least that way time doesn't really matter.