If you have a TD4 then it's just not going to happen at all. Forget it.
If it's an L series, then neat veg oil is too expensive to think about, and you need a good supply of waste oil to make that viable. Then you have a very ,messy process to filter it taking a lot of time and effort. Then you have to fit all sorts of heaters to the car costing £200+
Add to that increased servicing costs as the engine oil MUST be changed at least every 5k preferably 3k.
It's not as simple as you might think.