With great difficulty, is the answer!
For starters, you're sort of talking about two different disciplines. 'set lighting' could mean anything from Theatre or TV (and they themselves are two different animals again), 'festival lighting' could imply anything from live stage work to spending days schlopping through the mud with a million miles of festoon and catering supplies!!
TV/Film work is almost impossible to get into simply as it's such a small industry (that being said, take a look at the BBC recruitment pages as they sometimes surprise), heavily unionised and also very heavy on inbred terminology. If I asked you to go get me two blondes and a redhead, what are you going to come back with?!
Theatre is similar, but not impossible. Contact your local theatres (where are you based?) and ask to get put on their casual hands lists. The hours suck and the pay is crud, but it's experience and that's what it's all about! You'll need to be able to give them an incentive to take you on so make sure you emphasise your 17th and so on. However, also be prepared to wait a long time for the call - there's a lot of kids wanting to make it in and a lot of them are at college doing the stage tech BTECS. You also need to remember that these days most theatres outside the west end are on their knees financially.
'Live' work - so going out and about doing shows and things like that - find your nearest stage rental / production companies (again, tell me where you are and I might be able to give you some numbers) and get on their casual books. Expect even worse hours, loooong days and generally being taken advantage of at every turn, but it's a foot in the door. Be open and honest, say you're a qualified sparks (I'm assuming?) looking to learn the ropes. You'll almost certainly get landed with the lighting crew and they'll fill your head with every bit of nonsense terminology they can - NOBODY knows it all, it's just a BS act, don't worry.
Festival power - what are you like at fixing gensets, spilling diesel, IPAF essential, running MILES of cables and wiring up ceeforms?! Again, it's a bit of a closed shop as most firms tend to have summer crews that will do the same events year on year, it's only when people drop out that things tend to change. Find some local temporary power companies / generators near you and ask to get on their casual books.
Right.....now that I've thoroughly depressed you.... it DOES happen (you just need to be realistic). I lit my first stage show 30 years ago as a schoolboy, kept my hand in via the amateur dramatics scene, then one day just found myself in the right place at the right time, got offered a job with a rental company (having been working elsewhere after my apprenticeship), did it for a year or so learning the ropes and the lingo, went and worked for another company in a more senior role for another while, then felt brave enough to go freelance. Since then I've worked and toured with more 'stars' than I can remember, been around the world umpteen times and had the best of times. It's no real life for a partnered guy (it saw off my first marriage), it's VERY hard work and unlike anything else you've ever done. You meet and work with people in intense situations, for short spaces of time, you form life-long bonds and stories. You can also however get drawn into a life of alcoholism and chemical substances, too, so self discipline is required as much as technical skills.
PM me some more about your aims and I'll help if I can.