I've been writing a piece for winds and marimba, and I was looking to see if there was a proper way to notate this technique. I haven't seen an answer on music forums, but have heard many pieces use this. The bars are to be bowed across the corner with a fiberglass bass bow, and my current method is to use string up+down bowings. How, if there is a better way, should this be written?
It's fine and standard just to write arco or some equivalent in another language (just writing "bowed" is also fine). See this page for more stuff on marimba techniques.
I don't know that up/down markings would be necessary--would it make any difference to the sound produced? If not, don't include them; that would be unnecessary and only confuse the player, while making the music more cluttered.
It's time for you to purchase a good book on notation. You will not regret it. The Kurt Stone Music Notation in the 20th Century is a classic, although I also like the Elaine Gould Behind Bars: the Definitive Guide to Music Notation very much.