Begin by arranging the letters so they spell the out the plants you wish to mark. Depending on how many words you are making and how long they are, you might want several packages of letters.
Once the letters are arranged, place a dab of hot glue on the back of each (be generous) and start atthe top of the craft stick pressing them in place. Press firmly so they adhere well.
Keep working your way down spelling out the word of your choice.
Once done, you can apply a clear varnish if you wish to protect from the elements. Or, leave for a natural weathered look.