aim
Go at the static bots in front of the training room spawn and keep the cursor over the little circle on their heads. While doing so, move left-right, keeping the cursor as much in the middle as you can. Then add depth by going closer and further from them as you move. Keep the cursor as much in the middle as you can.
Then you can start to blink around, aim, track, blink, and so forth. Do that for 10-15minutes. Add 5 more minutes at the moving bots. Just track from one bot to the other, and the last few minutes start killing them, blinking, flicking, tracking, all at once.
If you have even more time for training, make a custom game. Add a medium Lucio, a high Hog, and an Ana (for hitbox diversity). Decrease their damage to minimum but still let them endanger you so you can feel some pressure. Get an easy Lucio on your side too.
Add the headshots only option, remove the hook, boop, and grenade. You can decrease and increase their speed too, and adjust it depending on your current tracking skills.
Doing this while improve you greatly even after one session. Then it becomes a habbit and the progression is a bit slower but noticeable.
Now, mechanically speaking. A better mouse would help.
I bought a DeathAdder and the big Razor pad so I can use a lower sensitivity. 1000Dpi with a sensitivity between 4 to 4.5. For heroes that require fast aiming like Tracer, Reinhardth, Pharah, even Junkrat, the sensitivity is about 0.5 higher than when I'm playing Soldier for example.
It takes a bit getting used too, but it happens in 1-6 days. You can find that most players use a DPI of 800-1000. From there, you can adjust the sensitivity by noticing how your aim tracks the bots when you do the first exercise above. If you always move the cursor just a bit more after switching directions, then your sensitivity is too high.
Focus only on the 'it's too high' part and decrease it gradually as you will feel the sensitivity being too low as you're still getting used to it.
A lower sensitivity will strain your hand more as you wont use only your wrist to aim anymore. You will have to move your whole arm instead. However, it is an immense difference in how much easier it is to track a target. A lower sensitivity gives you a !@#$load more control.
I used to play at 1200DPI and 7.5 sensitivity. It was insane. Now I'm feeling that 4.5 might be too much (at 1000DPI). If you cant buy a new mouse, consider buying one of the huge pads at least, cause you'll need a lot of extra room for moving your arm.
Extra tip: Practice flicking and tracking from the ground level to air level, as you'll need it with a lower sensitivity.
This is what I've done the past month and the result is that most games I have a much higher accuracy level. I never played McCree and now I see how strong he can be, averaging each game between 50-65% acc.
My only problem now is that I'm still a bit slower as I did not have time nor the mood to keep practicing so I can 'shoot per minute' faster. With McCree at least.
Extra tip 2: Practice with McCree the flicking and tracking part, then just kill as fast as you can with minimum shots with: Hanzo (For the projectile feel as you need to anticipate where your enemy will be, not where he's at), the switch to Soldier and go high ground over the moving bots, and finaly Tracer.
The time for a training (without a custom game) would be for me for about 30min.
15min tracking - McCree
5 min flicking - McCree
About 4min each of fast killing with Hanzo/Soldier.
And just 2 min of havocking with Tracer.
If you will train just with tracer, you won't reach her full potential.
The Headshoot feel with McCree, the Hanzo pattern movement prediction, nor the soldier feel of focused aim from mid'range (that helps you a lot when your closer with Tracer and it's a lot easier).