
Check-in time is different in each airport.
In the airport Boryspil it begins 2 hours 30 minutes and finishes 40 minutes prior to the departure.
When you buy the ticket please ascertain check-in time. Otherwise, your ticket can be sold if you don't appear for registration.
First of all you have to pass the passport and customs supervision and then you have to follow to the check-in clerk to obtain the boarding pass with your place and to register your luggage.
You don't have to pass customs supervision in "Green corridor" mood.
The boarding will be announced afterwards. When you hear this information you have to go to the gates indicated in your boarding pass, and there you have to pass the special control.
The country of your destination can have special rules, so when you buy the ticket ask the clerk about check-in procedure in that country.