There is a range of online resources providing advice on how to leave a gift in your will. Remember A Charity is a part of the Institute of Fundraising and and provides some useful advice on their website. MoneySavingExpert also have a useful article on writing a will for little or no cost.
If you're over 55, live in England or Wales, you can benefit from Free Wills Month which runs in March and October. If you don’t fit this age profile, you could consider Will Aid, which runs each November, where participating solicitors across the UK provide will-writing services without charge, and ask that you donate to Will Aid instead, supporting a number of chosen charities.
Free or heavily discounted will-writing services are also available through many Trade Unions, Employers and are even included in some home and motor insurance policies.