There are many ways to achieve different effects.
My favorite method is stippling paint over an item to mask the background. you can also ink over whatever you may be using.
you can use netting. It's available in different sizes and fabric stores.
also those bags that onions come in at the grocery store. Cut them open and ink or stipple paint onto the paper.
Hardware cloth gives a great larger square pattern.
silk flowers make a wonderful mask.
just some ideas :)