(Posted in 2 parts):
One of the many jobs I've done over the years was as managing director of a will-writing company, so you might expect me to disagree with OneEyedVic's suggestion that you need to use a solicitor. Surprisingly, perhaps, I'm not going to - but I would like to add a few notes of caution:
The first time I ever wrote my will (many years before I was professionally involved) I used the services of a local solicitor. At the time everything seemed in order. It was only when I came to do the job myself that I realised that there were loads of questions that the solicitor should have asked me but never did. (Although I have no formal qualifications in will-writing, I honestly believe that every client who had their will prepared by me received much more careful attention - and a more accurate document - than I ever got from that solicitor!).
When most people go to a solicitor they have a rough draft of their will in their mind. (e.g. "I want Fred & Mary to be my executors and I want my estate to be shared equally between my children Andrew and Sarah"). What they haven't worked out is the answers to the questions which the solicitor ought to ask - but probably won't. (e.g. what if Mary declines to be an executor? If Andrew dies in the same car crash that you do, who gets the money? His wife? His children? Sarah?).
So, what's my advice? First, head for your local library. Take out a book about writing your own will. Study it thoroughly. Prepare a draft of your will, taking into account all of the 'ifs' and 'buts' that the book puts into your mind. Then, and only then, go to a solicitor to ensure that everything really is in order.