Basically you simulate the sale based on the order book. Let’s say the order book contains the following bids ordered by price:

    Person A wants to buy 10 coins for 100 dollar
    Person B wants to buy 4 coins for 97 dollar
    Person C wants to buy 3 coins for 96 dollar
    etc

Now someone wants to sell 12 coins, he will sell 10 coins to person A and 2 coins to person B. The order book will now look like this:

    Person B wants to buy 2 coins for 97 dollar
    Person C wants to buy 3 coins for 96 dollar
    etc

Before dumping the 12 coins you could sell a coin for 100 dollar, after dumping you can sell a coin for 97 dollar. You could say the price has dropped 3%. Obviously this is a very naive calculation, but it should give you the basic idea.