How to find cheap flights
For me, the start of trip planning commences with Rome2Rio. You simply enter a start point (i.e. London) and an end point (Barcelona). As you can see from the below image, it will show you all of the major airports you can fly from in the UK (maybe there's one you hadn't considered which comes at half the price), as well as an estimate for driving, taking the train, bus, eurostar, ferry - basically if you can do it, they'll show you how. It also breaks down how long each method takes, which is super useful if you've got a lot of public-transport reliant planning to do.
If flying is shown to be the quickest, easiest cheapest route, my next stop is always SkyScanner
It has great features such as searching 'everywhere' (if you're not set on a specific place, but want to see what you can afford), as well as a feature to search for specific dates or by whole month, or even select from the cheapest month the website can find in the year. I think this is great, as most travel sites only give you a +-3 or 7 day window, which makes it near impossible to know where the cheap dates are. I also find most other travel sites to be really clunky and mess up if you try and fiddle around with dates. So, skyscanner is a winner for me. The algorithm retrieves prices from most of the leading airlines as well as travel websites and smaller companies. You get to compare all of this in one easy little space. It's super user friendly (pictures below to demonstrate the main features that I like).
Also note that you can do a search for multi-city stops, too, which is how I found out about TravelUp. In some cases, travelling to multiple destinations can be the same price as just the one, so it's worth looking into this if you have a couple of places in mind that you want to see. The site checks several airlines too. NOTE: it's worth picking out your destinations and days through sky scanner first, and then clicking to book on there as opposed to doing the initial search on there, which I found to be a bit sticky. BUT *WARNING WARNING* I've definitely found that some sites try to rip you off if you've visited more than once while you're trying to book, and then go and hike the price up. TravelUp ask for your details after you get a quote, including your email, and then increase the price when you get to the check-out page if you've done a couple of searches. I got around this by using a different email when I was definitely ready to go ahead and book, and - low and behold - the price was what I was quoted originally. Other sites might monitor your browser searches, so if the above trick doesn't work, try clearing your cache and starting again.
Also, do not get swept up by any too good to be true flight deals... stop and think, why? Check the website you're booking through on TrustPilot which gives a score rating from 0-10. It could be a scam site if the price is weirdly lower than every other site out there, and you'll probably see a low score, red, and lots of angry reviews from people who have had a bad experience. I think it's worthwhile reading through the comments of whichever site you book with (good or bad) just to understand the teething problems and avoid any unnecessary blunders which could dampen your trip.
Another tip, if each individual leg of your flight seems to be cheap, but when the return is selected it seems far more expensive than you'd have thought, try booking the two separately, as for some reason the price can at times go up when it's a return. Weird website logic.
I'm a fan of airlines which give you points. My personal favourite is British Airways who have an amazing awards programme called Avios, supported by using their American Express credit card which gives you a load of points for signing up. Ah, the ol' credit trap. But so long as you pay it off, why not?! You get reward miles for booking, reward miles for spending, and they really do go a long way. I've paid for several trips with my rewards now and I love it. Though, of course, I'm not sure BA necessarily falls into the category of cheap...
If we're talking about cheap airlines though, Ryanair has significantly improved and is by far the cheapest way to fly within Europe. The experience has become so much more user friendly, with much less charges and is now far more customer focused. Same goes for EasyJet who have recently modernised their planes, which are actually quite nice on board now.
Hope this little guide to finding cheap flights has helped. If you have any tips comment below!