You join employment agencies.
Although I agree with emori. It used to be like being self employed almost, in terms of autonomy, with none of the paperwork associated with being self employed. And you'd earn more per hour than the permanent staff, because you didn't get sick pay/holiday pay and could be told to leave without notice, if they didn't need you any more. Which seemed fair.
Last time I did it (many many years ago) the minimum wage had been introduced, so had holiday pay for all employees, even temps. The agency were after me signing a contract with them stating amongst other things that I wouldn't take time off for the duration of the placement. The "holiday pay" element was averaged out over what it would have been if you'd been a permanent employee and tacked onto your wages. So you'd get an extra 20p per hour or something, on top of what they were paying you, but no actual holiday pay between placements. There were less placements available too as companies cut back. Instead of one agency, I was registered with several at once to stay in consistent work.
Prior to NMW coming in there were plenty of people in basic jobs who didn't earn enough to live on even whilst working full time, they were forced to live either with parents or a partner, flat shares being for young people and HMOs not yet invented. Or become a single parent (you could claim housing benefit and income support, with no requirements to work, until the child turned 16).
Once employers couldn't get away with paying their most junior or menial staff peanuts any more, due to the minimum wage, wages came down for others to accommodate that without companies losing profits. They were no longer willing to pay agencies twice what a permanent employee would cost, so the result was the agencies earning a lot less and cutting the wages for many temp jobs down to minimum wage. Temping wasn't quite so much fun any more, after all that.
It is still an option though, if you don't need to work because you can live on one salary. You might find working full time but for only 6-9 months of the year preferable. So long as you save for the period when you're out of work and don't mind it being dictated to you when you can have time off (likely to be between placements only, if my experience is anything to go by). There are agencies for all different kinds of work.