You need a stamp, ink, paper and an envelope. That's all. I've used a clear stamp from Kaiserkraft (they have such beautiful stamps!) and a non acid waterproof ink.
I've put the clear stamp on the acrylic block and stamped carefully onto the envelope and the paper.
And easy piesy, here you go. Lovely stationary.
The only thing that's left now, is to write a letter. Done.