Sign with as many agencies as possible.
Some will withhold some of your pay so you can receive it over the summer. It is unusual to be offered work in September and it usually dries up between Easter and Summer.
I did a mix of long term supply and day to day.
The rules have changes about claiming expenses, basically you can't claim travel like you used to.
Day to day is probably the worst, you get called up to cover a secondary school, each classroom has a piece of paper on the desk asking the children to make notes and answer questions on page 52.
The students know you are only there for the day and they will play up. One thing I would do was start them making notes and then go around the class writing the time in the margin next to the last word written and then leave a note for the teacher.
It's good to have something up your sleeve for covering things like tutorials, I used to have a pack of cards with logic puzzles on, I'm also a sign language user and that has come in useful a couple of times.
If you cover secondary (I know you said you were primary but to start with take what is offered) most students will claim they do not have a pen, if you start off your day with a box of pens they will be gone before 3.00 pm, I got round this by having pens printed with 'stolen from Ms sashh' in a pencil case so I knew how many had been handed out, it was worth the cost and the pens always came back to me - they were also glittery so I could spot them.
Always take a packed lunch and flask of coffee. Most schools you need to have your finger print taken or an ID card to pay for food so don't expect to be able to buy something. On the other hand one school had unlimited hot drinks and compulsory free breakfast for staff and students.
If you agree to do half days check the timetable. I did long term supply at a school that had a late lunch and then just one period after lunch so the staff on day to day might be paid for a morning but actual work 4/5 of the day.
Overall I really enjoyed doing supply.
Good luck.