Lawyers will advise against writing your will yourself. Although a handwritten and signed will can be legally valid, they are not wrong to warn you. Unless you know the formalities of Wills and are careful with the language, there’s real risk your DIY Will won’t do what you want it to, or be legally valid at all.

A happy story of a beautifully hand-crafted Will is that of James Smithson, whose will created the venerable Smithsonian Institute, a treasure forever for the people of the United States:

“James Smithson wrote a draft of his Last Will and Testament in 1826 in London, only three years before he died. He died on June 27, 1829, in Genoa, Italy, where he was buried in a British cemetery. The will left his estate to his nephew, Henry James Hungerford, and stated that if his nephew died without an heir, the money would go ‘to the United States of America, to found at Washington, under the name of the Smithsonian Institution, an establishment for the increase and diffusion of knowledge ….’ After his nephew died without an heir, Smithson’s estate did come to the United States and a debate began about what this new institution would be.”
– http://siarchives.si.edu/history/exhibits/stories/last-will-and-testament-october-23-1826