What is the easiest/cheapest way to seal/stamp a brand in craft paper?
Simply put, that is a rubber stamp. That particular one appears to be professionally machine-made given the fine detail in the rendered image e.g. lines in the skull and the typography this would be expected. 

That does not mean you are forced to have one made. You can get very good results making your own. Although I doubt you could make the one you pictured without specifically designed machining tools.

There are many options for this as well. A very simple one would be to cut a stamp from a potato. You only need a potato, ink/paint and a knife for the some simple like a star. If you are curious this YouTube video would be a good primer. It even shows using cookie cutters to get more precise shapes. 

A realistic option that is more in line with what you show in your question would be to carve your own rubber stamp. Lots of tutorials out there for this one as well. I found this one from dearhandmadelife.com to be well written and full of useful pictures of the process. I will provide a basic outline of the process here. 

You need to following tools or variations thereof:


Carving rubber: this is what will become the stamp
Carving edge Tools: Search for rubber carving tools will get you the basic idea. Minimum you are looking for V-groove or U-groove edge tools. These would be better at removing material then your basic craft knife
Pencil and paper: Your initial design will be done in this medium
Mounting block: Optionally a block of wood, or something similar, to mount the carved rubber to. This will make the stamping process easier and give a uniform appearance to the applied stamp.


Use the paper and pencil to draw what you want the stamp to actually look like. No need to draw it mirrored or anything. Just exactly how you want it to appear. Fill in any space that you want to be stamped. White or blank areas will become the negative space of the stamp. 

The graphite from the pencil should transfer easily to the carving rubber. After that it is just a matter of carving out the blank areas in your rubber deep enough to prevent ink from adhering to it during the stamping process. You will also need to carve away an area outside the stamp as well for the same reason. 

If you wanted to mount it then cut off any excess rubber and glue it to the block. Stamp all the things! 

Your choice of ink will depend on your applied medium. Experiment. Standard ink pads should do the trick though. Depending on your product and placement you may need to worry about things like UV and the like.