What the others have said. I’m a native English speaker, but don’t really know what you mean by “Be Here Now”. Is it a command, like, “Come here now!” or is it more like, “Be mindful now at this moment”?
“Byddwch yma nawr” is like the first meaning. If it’s the second, you probably want something more like, “Bod yn ystyriol”, which is very different. You also need to check with a native speaker to be on the safe side as to the best translation of “mindful”.