I did it (for the second time!) in February.
They will call up more people than they need, as they have to allow for people dropping out/becoming ill, etc.
When you arrive you will go to the jurors' waiting area. In my case, this was a big room, lots of seating, and some tables for people wanting to do any work. There was also a café where you could buy your lunch and get teas/coffees etc.
You will also be asked about your availability beyond the 2 weeks that you're normally expected to be available for.
The jury officers will be in charge of allocating jurors to panels, and you'll be told what number panel you are in - and then it's a case of waiting for your panel to be called. In my case, I was waiting for all of Monday and Tuesday without being called - we finally got called in on Wednesday afternoon.
For each courtroom there were around 15-17 jurors in each panel - 12 of you will be picked at random. The others return to the jury waiting area, presumably to be allocated to another panel.
In the courtroom, you will be provided with paper and a pen to make any notes that you want. At various points the jury may be asked to leave the court, allowing the judge/barristers to discuss specific issues - you will be led in and out by an officer of the court. When you return (and on subsequent days) you will sit in the same place in the jury 'box'.
When all the evidence has been conveyed, you will be taken into a retiring room for you to begin your deliberations. The first thing you will have to do is to elect a foreman/woman, who will be responsible for delivering the verdict that you have all reached, to the judge.
There was no time pressure on us to reach a verdict. It's important that you contribute to the discussions that you'll have, even if you have a different view from others. The foreman/woman should also ensure that everyone has had an opportunity to say what they think - but all of the deliberation and discussion must be based on the evidence you've heard, and not just on your own opinion!
Once you have reached a verdict, you contact the officer in charge of your panel, and you'll be taken back into court for the foreman/woman to deliver the verdict to the judge.
At that point you will either be released back to the jurors' waiting room, or you may be discharged. In my case, we were discharged on the Friday after our case was complete, and I didn't have to attend for the second week.
You should receive a full information pack from your court before you go, which will also outline what expenses you can claim for.
As PP have said, there can be a LOT of waiting around, so make sure you take a book or something to do.