How do the armchair travelers among us create Michael Trapp’s Old World mash-up look in our own gardens without devolving into mishmash? Give the space some structure, Trapp suggests. “Keep clean lines and have some formality,” he advises, “and soften with plant material.” While adhering to that underlying layout, feel free to add architectural elements, like old stone edging or columns, or purchase a few antique pieces like a teak garden bench. Don’t be afraid to mix indoor décor, such as…