I want to bet against oil over a long term period (5-10 years, let's say). What is the best way for me to go about this, i.e what should I purchase and how?
It's difficult/impossible for individuals to carry long term options in the 5-10 year range like you're asking. Personally I wanted to go long oil, so I bought a couple of oil ETFs. You could find a "short oil" etf and hold that. These funds have access to the long term options you're looking for. The fund buys a stack of those options then you buy shares of the fund. Stay away from long term holding an "ultra short" or "3x short" or other highly leveraged short funds.
Be sure to research what the fund actually holds. Don't blindly trust the name of the fund or the highlights of the prospectus.