How to make a watermark on the image

[snowforest]

I want to make a watermark on the image with big transparent letters. 
Here is what I did:
I opened my image with gimp.
I created a text box. I typed the text of watermark.


Now how can I make this text transparent and rotate it so it becomes like a watermark?

Or tell me please how to do a watermark properly, if I am not on the right track...