Correct way to apply brakes
To reduce your speed significantly while on dry roads, I recommend front brake first and hardest, because the front wheel will have most grip. If you only want to lose a little speed (but more than soft-pedalling), and gently at that, e.g. in a group riding situation to prevent others running into the back of you, then rear-brake only.

On wet roads or loose surfaces you must avoid locking the front-wheel, so brake evenly and simultaneously front and rear. You have a chance of controlling a rear-wheel that is locked and skidding; that control is unlikely to extend to a locked and skidding front wheel.

If it is icy and extremely slippery, consider rear-brake only and don't go too fast to begin with!