A splatter brush is a premade (or you can always make your own ofc) brush that looks like, well, a splatter. You can get them here:
http://www.deviantart.comType in "Splatter brushes" in the search box, and then find a couple you like and download them. Then find the folder "Brushes" in your Adobe folder. It will be like, C:/Program files/Adobe/Presets/Brushes
Then drag the splatter brushes into the Brushes folder (You'll have to unzip them first ofc). Then start up photoshop again, and go into the brush tool. Right-click on the canvas and it will bring up a window. There will be a little arrow in the top right corner; click that and it will bring down a drop-down menu from which you select one of the brush sets you downloaded.

"Now when I do this and create a clipping mask, only 1 or 2 pieces of the brush will blend and do something, the rest will stay white and be unchanged"
I think that's because you're dragging the grippled layer in the wrong direction. It's only so big and if you drag it too far off then the normal color of the splatters will show through.
By the way, ALWAYS use a new layer for these

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"How do I create a light source?"
Reset your brushes (drop-down menu, select Reset Brushes) and select a large (about 250px) soft brush. Make it a very bright color, white or another bright color that fits your signature. Then look at your stock and find out which way the light is coming from on it. Once you've found that out, then, in that direction, click off the canvas with the brush. You click off the canvas because if you clicked on it, the light source would be too big. This very large brush clicked off the canvas is good for a very gradual light source.
Then once you've done that you can use a smaller brush (about 100px) in a slightly brighter shade, or if you used white before then just white, and click very close to the canvas this time (but still slightly off).
"How do I create scanlines?"
Engage made a picture to help people who didn't know how, but the dumbass deleted it off his photobucket ;(
It's basically a pattern. New document, make it 3x3 pixels. Then take the pencil tool, 1px, and make a diagonal line from one corner of the canvas to the other. Then edit > define new pattern. Then in your signature's document, open it and fill it in.
"How do I create a focal mask?"
In the layer window, on the bottom next to the fx button etc., there will be a button that looks kinda like a camera. That's it

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"This is what I have so far, is it going to work for the rest? And do you think it will work nicely with the rest?"
You might want to start over and use scatter with your smudging

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Hope I helped!
Cheers