If you're talking about legally leaving behind instructions on what to do, I think a will is the way to go. I think you can get "do-it-yourself will" books, I've seen them for sale at government service buildings before. This might vary by country though.
In some places, "holograph wills" are legally valid. They are wills completely written in your own handwriting and signed by you. Often there might be limitations, and questions can be put into authenticity, your mental state when you wrote the will (this might come into play especially with suicide). If it's just like "I want to be cremated" I doubt anyone would contest it though, unless you have crazy religious people in your family who would want control over your body so you can be disposed of in a way that fits their spiritual ideas of what will get you to the good place. I think contesting wills and these types of legal issues usually only come into play when there is a large estate with funds and assets to be divided up that scummy family members want to fight over.
Based on what you've told me, a holograph will should just be fine... Even if not legally binding I imagine it should be honoured unless your family are assholes. Just write out what you want done with your body after you die, or what to do with belongings.
If you want to know like, how to actually organize a cremation and call up a cremator to reserve a spot in their oven, can't help you there.