Finding the cheap european flights can be quite a time intensive process. The reason is that there are a ton of low cost airlines that are not listed in the major flight search engines. To find the cheapest flight, you'll need to be a bit creative and put in some effort.
Here is the most effective way to do this:
Check a major flight search engine to find the cheapest flight with a regular airline. I prefer to use kayak.com or skyscanner.com
Find out which European low cost airlines serve your departure and destination city. The easiest way to do this is to use Fly LC. This website enables you to select a departure and arrival city and shows you which low cost carriers serve this route.
If you don't want to rely on third party information, the best thing to do is to check the website of the relevant airports. The information provided here is always 100% up-to-date. It's more work, but if you have the time it can be worth doing.
Check the prices of your flight on the website of the low cost airlines that serve your departure and destination city.
What if there is no low cost airline servicing my route?
If you can't find a low cost airline servicing your route, you still have a few options left.
Repeat step 2 and 3 for airports nearby your departure and destination city. It might be cheaper to first take a train or bus to a nearby city and fly from there instead of the airport closest to you.
See if you can create a two-flight itinerary using two different low cost airlines. The best way to do this is to find out if there are airlines servicing your departure and arrival city with a common hub.
Let's say you want to go from Split (Croatia) to Reykjavik (Iceland). No low cost airline serves this route (in fact, no airline does). Using a major search engine like kayak yields no acceptable results, prices $500+ with terrible connections. There is an affordable option though. You can fly easyJet to London Gatwick and WOW Airlines from London Gatwick to Reykjavik. Total price under $300.