How do I split a stack of items?

I tried the search function, but either I have really bad syntax or its not been asked. I'm guessing the bad syntax *sigh*

Anyways, I've been accumulating items from salvage, just learned that its worth a LOT more than just selling to vendors, but I have no idea how to split the stack. If someone wants to buy 5 wood and I'm sitting on 20, how do I break it up?

Any help would be greatly appreciated. Thanks.

When you open the trade window with another player, you select the quantity of the item you wish to sell. Better than splitting, in my opinion.