How to get Algae to grow in slate/rock

[Moobs99]

So I would like to get some algae to grow on my granit rocks and slate to add to the defect of my ciclid tank,
I was thinking of filling a spare 3ft tank filling it with water from my tanks and leaving in the garden for a few days, then add the rock on maybe day 5/6 and leaving till I get the right growth, is that's the best way or is there an easier way ? This is the effect I'm after

 

[Tetra1990]

You have the right idea. Leave the spare tank in the sun for awhile with the slate/rock in it. When the right growth is accomplished, transfer to the other tank.