This is how I make my signatures currently. With Photoshop.
(My first tutorial, go easy on me. Haha)
First, make the canvas whatever size you'd like. Make white (or your preferred color) as the background.
I'd recommenced greatly, to download some brushes. Some Grunge Brushes, Swirl Brushes, Smokey Brushes, Splatter Brushes, etc
(I downlaoded mine from Deviantart. Here
]. After you have the brush you'd like, start brushing your canvas whatever way you like.
Then add some color. I did this by using a smokey-like brush, with the color green. I brushed which places I would like to be green. Or you could simply just go to Adjustments > Selective Color or Image > Adjustments > Color Balance.
I then go to Adjustments > Exposure and start playing with that until I find something I like.
With a Render of my choice, I add it to my work. I then add a nice smudge/blur effect. (To do this you duplicate your Render's layer then go to Filter > Blur > Motion Blur. You can play with that, till you're satisfied. Then make sure the orignal layer of the Render is above the blurred layer. It should have a kinda smudgy/misty look around it.)
I then use some more brushes to add to my work. Here, I added swirls and some splatter. Once I think I'm finished, I add some text.
Then I crop it a bit, and add a tiny bit more Exposure if needed. (Adjustments > Exposure) I do anything else I see needs to be done, or just touch it up nicely.
Then, optionally, I add a border. Image > Canvas Size > Make sure the box is clicked on relative. Then switch to Pixels, and I usually put 2 or 3.
Then I'm pretty much finished.