electric engine cooling fan soft start

I am looking for a very simple diagram of a soft start system using a relay, temperature sensor and most likely a MOSFET. Basically, I need to soft start an engine cooling fan the easiest, cheapest way possible. Can someone provide a diagram and some detailed instructions for my project, Thank you for the help.

The cheapest is probably a PICAXE 08M2 ($3) that you program to PWM ramp up the MOSFET and then switch in the paralleled relay when at 100% PWM. It may also be the easiest... But both above are subject to POV, plus a PC & serial interface for programming, and the programming. (But heaps of PICAXE stuff is available free of charge.) I argue for the PWMing of the MOSFET to overcome the need for major heatsinking if using the MOSFET in linear mode. I have been meaning to do the same for electric window winders & fans etc but haven't gotten a round tuit yet.