Want to print some credit card sized images on A4 paper? Use the following templates to set up your document in your favorite image editor. Save the images by right clicking -> "Save image as".
For short-term usage, I've had good results laminating A4 and then cutting out the individual cards. Just don't expect these to last, as the edges will start to fray eventually.
These outlines should get you 8 CC sized images on a single piece of A4 paper. Print them directly on the paper, or simply use them as a temporary guide.
You can use the following image as a (transparency) mask, in order to limit your card design to be exactly CC sized. Example: in Krita, import the image as layer, right click it and "Convert to Transparency Mask", and then use the mask layer on a group, background layer, et cetera.
This mask is slightly wider than the previous one, giving you a bit more room for error when cutting/slicing up the printed paper.