How to create a Japanese Pen Name?

Question:

I was reading a manga called Bakuman and these two kids made the pen name: Muto Ashirogi (first name, last name). How do these Mangaka or ANY Japanese person create these odd pen names? Ashirogi, Kawaguchi, etc. Are they actual Japanese surnames?

Answer:

I have never heard of Ashirogi, but Kawaguchi (川口) is an extremely common surname.

Names in other manga often use randomly combined Japanese sounds and/or words.

Ichigo (from Bleach), for example, could mean "strawberry" in Japanese (though the kanji they use is different [一護]). His last name, Kurosaki [黒崎], is a common surname and place name.