It is possible for you to make your Will yourself. You can write it on the back of a cereal packet if you want – as long as it is signed and witnessed correctly, and depending on what you have written, it potentially could be a legally valid Will. As a slight step-up in quality, you could purchase a DIY Will kit. This will give you some guidance as to what the Will should contain.
Before taking this route, consider whether taking a “cheap” option of the self-made Will is the right action for everything you have worked for all your life. Considering how relatively inexpensive it is to get your Will professionally made with expert guidance, there is very little reason not to get some help.
Look at the shoes on your feet right now. You could make them yourself if you really wanted to. But if you did, I imagine (unless you are an expert cobbler) that the shoes you would produce would not be nearly as good as the ones you are wearing. It would take you a long time. You would get stuck along the way and would probably give up. Or you could do what you did do – you went to the marketplace and bought a pair that had been professionally produced.