This is easy!
You need an N channel fet, even a TO-92 small plastic one will work, like the ZVN4206A from Digikey (here in the USA). A TO-220 power tab will have more current ability.
The source lead goes to ground, the gate goes to your digital out. You don't need a gate resister unless going very fast, like 50KHZ, for tailoring the gate drive waveform. In that case use a FET gate driver chip, an inductive bead helps too.
The drain lead goes up to one side of the relay coil, the other side of the coil goes to V+. The diode, which should be a fast recovery type if doing PWM, has the band going to V+, the unband going the drain. The diode is across the coil.
Some people do not use the diode, but instead a snubber. This allows faster switching, because there is not the "freewheeling" current being recycled thru the coil. With very sensitive, high resistance relay coils, you will actually see the relay stay on momentarily after shutting it off due to this!
Darlingtons are old hat, they never completely saturate and have too high of an on voltage. I don't use them anymore. Or vacuum tubes. Or spark gaps....