Aim - The best way to improve your aim, is to go on 128 HS only servers (preferably around 30 - 50 ping). Do not spray, do not pre-fire everything. Aim first > then one tap shoot.
Spraying - Go on an empty server (Create your own), get yourself an AK / M4 (Try to practice with both, at least 5 - 10 minutes with each) and spray at the wall.
Burst and Side stepping - (Video from WarOwl)
30 minutes every day and you should see an improvement in no time.
NOTE : This might not be the best way, but it has worked for me.