It's good to report the need for assistance when buying a plane ticket. European rules shall operate at least 48 hours before scheduled departure.
Order assistance with the airline you're flying to and state that you are a blind or weak-sighted passenger. Describe which part of the passage through the airport you need to cover. Remember to mention guide dog, transfer or help after arrival.
At Prague Airport, the assistance service includes assistance for check-in, security check-in and airport transfer during departure and arrival.
Sources: EUR-Lex, Airport Prague