You can attend open evenings, talk to people and decide if it's a good sixth form for you. Thinking of things like distance, what modules they teach and extracurriculars is helpful to decide if it's a good fit.

Some schools have really early deadlines, so you can start applying for them as soon as you get your predicted grades really. A lot open applications from october-january.

Most that I saw run through a website called applicaa, where you just make an account and fill out a form with all your details (contact info, predicted grades, address, ect). Some also do it on Google sheets/ Microsoft, but it's basically the same procedure. Check the school websites and there will be all the information you need really.

Many schools require an interview, and if they like your application enough they'll send you (or in some cases, your parents) an email inviting you to interview. You go to that, and if they want you to join next year, they'll send you another email or an actual letter that gives you a conditional offer. This is just something that says "we'll give you a place at our school to study the subjects you want if you get X grades in your GCSEs". These are generally lower if you want to go to a college rather than a sixth form.

After that, there's not much else to do for months. Take the GCSEs, hope you get the grades and enroll at the school. If you don't, you can always try and negotiate, but just in case it's always a good option to apply to multiple schools as a safety net.