I've done quite a lot of supply.
Don't expect to work in June and July - you might get some work, bit it is rare because lots of children have gone on study leave so staff are redeployed.
Sign up with every agency you can.
Be prepared for morning calls, you get a call at 7am and you have to be out the door in about 30 mins - if i'm doing that I have shower, breakfast etc but sit in joggers until I get a call, that might not happen.
Once you get 'known' you get more work, schools/colleges will call the agency and request you.
Be prepared to teach anything and have something in your bag in case there is no lesson planned. I have a set of logic puzzles, they are not exactly the best lesson but at least the kids think and do something.
I've 'taught' photography, maths, sociology, ICT, emplyability skills ...
Depending on your subject(s) you may get long term placements, I did January to Easter in one school. But they can sign you up for things like that and only give you 1 day notice to terminate it.
Also if you have a particular skill you might find yourself teaching just that, so lots of teachers can teach health and social care, but not all of them (including the specialists) can teach anatomy and physiology - I have spent months teaching nothing else, just that BTEC unit and then on to the next place to teach it again.
Be prepared to travel. The rules have changed for claiming expenses, a couple of years ago I spent 6 weeks in a hotel because it was too far from home to travel, now you would have to fund that yourself and claim it back via tax returns.
Most agencies will have a list of places you will/will not work. I won't do work in the two schools closest my home, I don't want to teach kids who know where I live.
I will do special schools, and FE but not primary. I will do long term, short term and 1/2 days. Be careful with half days though, lots of school have 80% of the teaching before lunch so they are getting almost a day's work out of you for half the rate.
Think about tax credits, as a salaried teacher you can claim them over the summer, as a supply teacher your contract has ended so your tax credits claim has to end too.