I believe those huge topiaries at theme parks are all done around metal frames. Seems to me I caught a glimpse of one.
Jim

Ed,

I believe you are referring to large metal frames, covered with wire, filled with peat moss and other soilless mixes and imbeded with plants. Mondo grass for eyes or hair, succulents, creeping plants (in Asia, creeping fig is ideal)

It's a lot of maintenance. Plants require trimming every week, liquid fertilizers and replacements. Most displays have greenhouses with supplies readily available to replace dying plants to keep the display looking refreshed. Watering is carefully monitored.

Best advice is to start off small and build up as your confidence and skill grows!

Many more managable topiaries are a better way to start!

Make a thing, stick them on it.
A Ball'O Bromiliads was a popular hanging plant when I was a kid (people used Hurricane Balls, which were freaks of Hurricane Camille. Plant matter was compacted together and you could grow things on it.)
So you could make an object and cover it with bromileads. Or succulents. Or lots of little cacti.
A lot of people here still have a wall'O Bromiliads where they are growing in driftwood instead of Hurricane Balls.
Give it a go.
Not a true topiary, but hey, it's fast and colorful.

YOu know drasaid, you just gave me an idea. Not a wall of bromeliads but a leaning log covered & dripping with hens and chicks and other succulent. I've been pondering on my garden project for this season and you've just helped. Thanks
Ianna

Believe it or not, there are three types of topiary... It sounds like you're interested in the "stuffed" type. I have a 5-foot tall stuffed topiary in my back yard. Yes, it takes a lot of work, but it's soooo cool. First, you need a rust-proof frame. I had a custom frame made. For a larger topiary, you'll need to run irrigation lines through it (to keep the spaghnum moist from within). I have mine hooked up to my irrigation. Fishing line is used to maintain the stuffing in place. Hair pins (i.e., bobby-pins) are used to secure vines against the topiary surface. The pins should be pushed straight in to prevent them from being visable. Fertilization is important because spaghnum doesn't contain nutrients -- typically 2X a month at half-strength. For smaller portions of the topiary, overhead watering will be needed.

The method with the least maintenance is to make/have made a welded steel frame in the shape you want, then buy the plant and set it in the middle of the topiary frame. Pick a plant that will eventually reach the size of your chosen frame.