Newbie Hatching Some Eggs - 21 Day Countdown?

[LovelyBantam]

I would love any advice for incubating and hatching eggs as I am a complete newbie in doing so. My Eggs are planned to hatch about the 18th of August. My flock is mixed so I don't know what I'll really end up with. Any advice is very appreciated.

 

[Chickens 235]

It usually takes 21 days for a chick to start to hatch from 6-12 hours to get completely out of the egg