Trim stems of greenery leaving 1-2" of stem to insert into the wreath. For eucalyptus, 6-8" overall.
Work your way around the wreath inserting stems in the same direction as existing stems, tucking them under the evergreen until they feel secure. Keep in mind you can readjust spacing any time. That's the best part - you get easy do-overs!
Trim flower stems or thistle to 4" tucking them in at varying widths to stagger, again, in the same direction as existing stems. Insert them until almost flush.