My trainer actually has had us doing a lot of cavaletti/ground pole work with all our horses, from the greenies to the intermediate/advanced eventers.
Basically she has set up something similar to this (pretend that I have some drawing skill with lines) >
The green poles are set at, basically, walk distance (trust me, you don't walk them) and the red poles are set slightly longer than regular trot poles. The blue poles are set on a 15m circle angle.
First just start off going over the red poles. The goal here is to maintain a consistent relaxed contact and to encourage your horse to really lift through their back, push from behind, and lengthen their stride through the poles while maintaining their self carriage. Do them both ways until you feel this consistently.
Then go through the green poles on their own. Here you want to achieve a shorter, more collected stride while again maintaining the self carriage and impulsion from the hind end as well as the relaxed connection through the horse's mouth. When the horse has started working consistently through these you can start to string the red and green poles together.
Start off in the easier direction of green > red poles. You want to achieve the shorter stride BEFORE reaching the cavalettis, just as you want to have your horse moving forward into a more lengthened stride BEFORE they get to the red poles. Basically you're working on a) adjustability, b) consistency, c) suppleness. After you've gone through green > red several times successfully you should go ahead and do the red > green. Keep in mind this is harder. You have to maintain the impulsion as you shorten/collect as well as maintain the self carriage and lightness. If you need to make a small circle in between that's fine too, just try to work towards straight from one to the other.
The circle ones you can add in after going through them a few times on their own. The key is to keep your circle consistent. Don't come into the poles on an angle. Just ride your circle as if the poles aren't there. Your horse will figure out where to put their feet. This is a great exercise for nervous horses or horses that worry easily (we have a few of those) because they can stress themselves out before or after and they'll just keep coming back around to it. Eventually they get into the rhythm and relax into the exercise.
You can string all of the sets together in multiple ways. You can add in 10m canter circles before and after sets of poles for more advanced horses, as one idea, as long as you trot through the poles, since they're at trot distances.