You have two tanks, one for the straight VO, and one for diesel. You start the engine on diesel. Once the engine is running, some method for heat is sent to the tank containing the VO to start heating it up. Most common is to use the radiator water lines run through a "coil" or some type of heat exchanger in the VO tank. Once the VO is heated enough (thin enough) to use, you switch over to the VO tank and run on that until it's time to shut down. Then you switch back over to the diesel tank to flush the fuel lines and injectors (you don't want VO sitting in those lines or injectors or it will gel). Then you shut down.
A commercially available conversion kit will usually have all the parts you need for the conversion, AND some will offer automation of the switching. The automated systems will monitor the temps and do the switching from one tank to another for you.
While I like the technical feasibility of it, here's my problem with a conversion kit. If you are technically inclined enough to build your own, then great. However, if you buy an off-the-shelf kit, they can be very pricy, almost the same price as an entry level bio-diesel processing system. So why not put the money in a bio-diesel processor and be able to use that fuel in any diesel vehicle, not just one vehicle that has a conversion kit?
I have a heat exchanger in the front tank to warm the veggie oil. If itís less than 70 F out veggie oil does not flow very well.
I have 4 heat systems to heat my oil. First is in the fuel tank. Itís a transmission oil cooler that has engine coolant flowing through the cooler and it sits on the bottom of the veggie fuel tank. I have 18 feet of hose on hose fuel line. Coolant flows through one hose and veggie oil through the other. 12 volt 200 watt heater that raps around my veggie oil fuel filter. Arctic Fox fuel heater that uses coolant to heat the fuel.
I have been running an F250 with a Goldenfuelsystems.com system since March 2008. I ran it during cold days (~20 degrees) without problems. Their system uses tubes from the coolant system to keep the oil hot. I didn't see any lag in power during the cold months. I am now getting into the warm months and am enjoying the longer run time because the oil is already heated in the black tank in the bed of the truck.