The only tip I can offer is just squeeze and release with the tips of the grass in various directions so all of the blades of grass won't be headed in the same direction. I have a baseball cake in my pics and you can kind of tell what I mean from that. HTH. Good luck with your project, and be sure and post a pic. 
More companies make tips besides Wilton. I also buy Ateco tips. For grass I prefer the larger 334, Wilton's standard 234 is too small and takes too long on bigger cakes.
http://www.atecousa.net/
Click on Learn and look at the book of tips and what they do.
The grass tip is awesome! It's so easy and fun to use and people are so impressed when there is grass around their cake!
Just do a little practice on a piece of wax paper before you start. You will have the hang of it in no time. Just squeeze and release!
Good luck!
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