I know this is a really beginner question, but I was wondering if someone could tell me how to apply Heki grass to a base? I just assumed I'd glue it down but it looks a bit more complicated than that. Thanks!

No, just spread some 50/50 water glue and paste it on teh base with a paintbrush. then lay the heki over it.

tsu, tacky glue? is it going to need a 50/50 woodglue/water like the flag?

White glue and water - spread it out over the base. Lay the grass over the mix, pat down lightly so there is good contact. That's it. You may need to go back and touch up ground color depending on what 'shows through'
Actually - you'll want to be sure the the base is painted a ground color 1st just in case the Heki doesn't cover 100%. Its easier to put the color down before the grass rather than after.