Sources
Planet Renders
deviantART
Steps
Step 1
How to open C4D
PNG type of C4D (transparency)
File->Open...->Drag the whole C4D in the sig->Set to "Color dodge" or "Linear Dodge (Add)".
TIF type of C4D (not transparency)
File->Open...->Selection->Load Selection...->Set the drop-down window to "Alpha 1"->Click "OK" button->Drag the C4D in the sig->Set to "Color dodge" or "Linear Dodge (Add)".
Step 2
Delete unwanted part
I recommend you to use soft round brush to erase the unwanted part. And remember to adjust the opacity.
Example of C4D Sigs,
Taken from: The Curry + Vervex + Friends Happy Shoppe!
Extra
C4D Bubble Pack by Orrin - Here
Uploaded times - [1, 2, 3]
Please remember that it's your own design. Simply merging stock with a C4D is not a good way to design. Remember that it's about your own creativity!
Please learn how to apply the right C4D with right position in your sig.
